<<

AMERICAN MATHEMATICAL SOCIETY

Research in Collegiate Education. V Annie Selden, Tennessee Technological University, Cookeville, Ed Dubinsky, Kent State University, OH, Guershon Hare I, University of California San Diego, La jolla, and Fernando Hitt, C/NVESTAV, Mexico, Editors This volume presents state-of-the-art research on understanding, teaching, and learning mathematics at the post-secondary level. The articles are peer-reviewed for two major features: (I) advancing our understanding of collegiate mathematics education, and (2) readability by a wide audience of practicing mathematicians interested in issues affecting their students. This is not a collection of scholarly arcana, but a compilation of useful and informative research regarding how students think about and learn mathematics. This series is published in cooperation with the Mathematical Association of America. CBMS Issues in Mathematics Education, Volume 12; 2003; 206 pages; Softcover; ISBN 0-8218-3302-2; List $49;AII individuals $39; Order code CBMATH/12N044

MATHEMATICS EDUCATION Also of interest .. RESEARCH: AGul

Teaching Mathematics in Colleges and Universities: Case Studies for Today's Classroom Graduate Student Edition Faculty Edition Solomon Friedberg, Boston College, Chestnut Hill, MA This series is published in cooperation with the Mathematical Association of America. CBMS Issues in Mathematics Education, Graduate Student Edition: Volume I 0; 200 I; 67 pages; Softcover; ISBN 0-8218-2823-1 ; list $16;AII individuals $13; Order code CBMATH/1 ON044 CBMS Issues in Mathematics Education, Faculty Edition: Volume I 0; 200 I; 158 pages; Softcover; ISBN 0-8218-2875-4; list $29; All individuals $23; Order code CBMATH/1 O.FN044

The Mathematical Education ofTeache rs This series is published in cooperation with the Mathematical Association of America. CBMS Issues in Mathematics Education, Volume II; 2001; 145 pages; Softcover; ISBN 0-8218-2899-1 ; List $24;AII individuals $19; Order code CBMATH/1 I N044

For many more publications of interest, visit the AMS Bookstore www.amsbookstore.org

I-800-321-4AMS (4267), in the U.S. and , or 1-40 1-455-4000; fax: 1-40 1-455-4046; email: [email protected]. American Mathematical Society, 20 I Charles Street, Providence, Rl 02904-2294, USA ~104 International Mathematics· Research Notices

Editors Website: http://imrn.hindawi.com Morris Weisfeld IMRN provides very fast publication of research articles of high current interest in all Managing Editor areas of mathematics. All articles are fully refereed and are judged by their contribution to advancing the state of the science of mathematics. Issues are published as frequently as Dan Abramovich necessary. IMRN is expected to publish 80± issues in volume 2004. The articles ofiMRN are Enrico Arbarello reviewed/indexed in COMPUMATH Citation Index, Current Contents, lSI Alerting Services, Joseph Bernstein Mathematical Reviews, Science Citation Index, SciSearch, and Zentralblatt fUr Mathematik. Enrico Bombieri There are no page charges. Submissions are made by email to [email protected]. Richard E. Borcherds New print subscribers shall receive a free copy of all back volumes, i.e., volumes 1991- 2003. Alexei Borodin New and current online subscribers shall receive "perpetual" online access to volumes Jean Bourgain 1991- 2004. Contact [email protected] for more details. Marc Burger ARTICLES Tobias Golding FORTHCOMING Corrado DeConcini • A Cuspidality Criterion for the Functorial Product on GL(2) x GL(3) PercyDeift with a Cohomological Application, Dinakar Ramakrishnan and Song Wang Robbert Dijkgraaf • A New Proof of Steinberg's Fixed-Point Theorem, G. I. Lehrer S. K. Donaldson • A Stacky Semistable Reduction Theorem, Martin Olsson Yakov Eliashberg • Associated Primes of Local Cohomology Modules and of Frobenius Powers, Edward Frenkel Anurag K . Singh and Irena Swanson Emmanuel Hebey • Bounded Geodesics in Moduli Space, Dmitry Kleinbock and Barak Weiss Dennis Hejhal • Gap Probability in the Spectrum of Random Matrices and Asymptotics of Polynomials Helmut Hofer Orthogonal on an Arc of the Unit Circle, I. V. Krasovsky Gerhard Huisken • l -Functions of Exponential Sums over One-Dimensional Affi.noids: Newton over Yasutaka Ihara Hodge, Hui June Zhu Michio Jimbo • Minimality and Nondegeneracy of Degree-One Ginzburg-Landau Vortex as a Hardy's Kurt Johansson Type Inequality, Manuel del Pi no, Patricio Felmer, and Michal Kowalczyk Masaki Kashiwara Kiran S. Kedlaya Carlos Kenig Sergiu Klainerman Toshiyuki Kobayashi Maxim Kontsevich Igor Krichever Shigeo Kusuoka Gilles Lebeau Joachim Lohkamp Nikolai Makarov Yu. l. Manin Barry Mazur Haynes Miller Shinichi Mochizuki Fabien Morel Michael Rapoport N. Yu. Reshetikhin Igor Rodnianski Peter Sarnak Freydoon Shahidi Stanislav Smirnov Michael Struwe G. Tian John Toth Takeshi Tsuji David Vogan Dan Voiculescu Hindawi Publishing Corporation , 410 Park Avenue, 15th Floor, #287 pmb, New York, Andrei Zelevinsky HINDAWI NY 10022, USA; Fax 1-866-446-3294 (USA, toll-free) Maciej Zworski NEW f6 FORTHCOMING from Birkhiiuser

Mathematical Theory Regularity Theory for Linear Algebra Second Edition of Diffraction Mean Curvature Flow J. H. KWAKand S. HONG, both, Pohang University of A. SOMMERFELD (Author) K. ECKER, Freie Universitiit, Berlin, Germany Science and Technology, Pohang, Republic of Korea R. J. NAGEM, Boston University, Boston, MA; M. ZAMPOLLI, NATO SACLANT Undersea Research Centre, La Spezia, Italy; and This work focuses on the motion of surfaces whose ':41ogical development of the subject .. . all the G. SANDRl,Boston University, Boston, MA (Translators) normal velocity at every point is given by the mean curvature at that point; this geometric heat flow process important theorems and results are discussed in A. Sommerfeld's Mathematische Theorie der Diffraction terms of simple worked examples. The student's is called mean curvature flow. Mean curvature flow and marks a milestone in optical theory, full of insights that related flows are important tools in mathematics and understanding is tested by problems at the end of are still relevant today. In a stunning tour de force, each subsection, and every chapter ends with mathematical . This work presents techniques in Sommerfeld derives the first mathematically rigorous the study of singularities of mean curvature flow and is exercises. " -CURRENT SCIENCE solution of an optical diffraction problem. This com­ (REVIEW OF THE FIRST EDITION) largely based on the work of K. Brakke, although more plete translation, reflecting substantial scholarship, is recent developments are presented as well: for exam­ Acornerstone of undergraduate mathematics, the first publication in English of Sommerfeld's original ple, relations to regularity theory for minimal surfaces, science, and eHgineering, the fundamentals of work. The extensive notes by the translators are rich in as in Allard's and de Giorgi's work. linear algebra are given a clear and rigorous historical background and provide many technical presentation in this text. With emphasis on both details for the reader. 2004/144 PP., 16 ILLUS./HARDCOVER/$ 129.00 ISBN 0-8176-3243-3 theory and computation, the authors cover linear 2004/190 PP., 381LLUS./HARDCOVER/$54.95 PROGRESS IN NONLINEAR DIFFERENTIAL EQUATIONS AND THEIR equations, matrices, determinants, and vector ISBN 0-8176-3604-8 APPLICATIONS, VOL 57 spaces, and continue on to complex vector spaces, PROGRESS IN MATHEMATICAL PHYSICS, VOL 35 inner products, quadratic forms, and Jordan canonical form. Approximation Theory Semiconcave Functions, From Taylor Polynomials to Wavelets KEY TOPICS AND FEATURES: Thorough treatment of the basics, with a particular focus on linear Hamilton-Jacobi 0. CHRISTENSEN and K. L. CHRISTENSEN, both, recurrence, difference, and differential equations Equations, and Technical University of Denmark, Lyngby, Denmark • Avariety of applications of linear algebra to This concisely written book gives an elementary intro­ diverse fields, such as economics, physics, and Optimal Control duction to a classical area of mathematics-approxima­ • Rich selection of examples P. CANNARSA and C. SINESTRARI, both, Universitii di Roma tion theory-in a way that naturally leads to the modern and explanations, with a wide range of exercises at 'Tor Vergata', Roma, Italy field of wavelets. The exposition demonstrates the the end of every section. This text is a exposition of the theory of semiconcave dynamic nature of mathematics and how classical This second edition includes substantial revisions, functions, and of the role they play in optimal control disciplines influence many areas of modern mathemat­ new material on minimal polynomials and diagonal­ and Hamilton-Jacobi equations. The first part of the ics and applications. The emphasis is on ideas rather ization, and several illuminating figures and book covers general theory, encompassing all key than technical details and proofs. diagrams. Valuable for theoretical and applied results and illustrating them with significant examples. KEY TOPICS AND FEATURES: Description of wavelets in courses, this work is also ideal for self-study. The latter part is devoted to applications concerning the words rather than mathematical symbols • Numerous Bolza problem in calculus of variations and optimal exit­ 2004/APPROX 436 PP., 40 ILLUS./SOFTCOVER classical, illustrative examples and constructions • $49.95 (TENT.)/ISBN O-B176-4294-3 time problems for nonlinear control systems. Practical applications of wavelets to digital signal 2004/APPROX. 278 PP./HARDCOVER/$129.00 processing and data compression • Excercises. ISBN 0-817 6-4084-3 PROGRESS IN NONLINEAR DIFFERENTIAL EQUATIONS AND THEIR 2004/APPROX 168 PP., 40 ILLUS./SOFTCOVER/$29.95 (TENT.) APPLICATIONS, VOL 58 ISBN 0-8176-3600-5 Complex Numbers APPLIED AND NUMERICAL HARMONIC ANALYSIS from A to ••• Z Spatial Patterns T. ANDREESCU, University of Wisconsin, Whitewater, WI,· and An Introduction AMC, do University of Nebraska, Lincoln, NE; and Higher Order Models in D. AND RICA, Babes-Bolyai University, C/uj~Napoca, Physics and Mechanics to Continuous· Time Complex Numbers from A to ... Z is an introduction to this L. A. PELETIER, University of Lei den, The Netherlands; and Stochastic Processes fascinating subject, with a particular emphasis on key W. C. TROY, University of Pittsburgh, PA Theory, Models, and Applications to concepts and elementary results concerning these "This book is about a new family of higher-order scalar Finance, Biology, and Medicine numbers. The reader learns how complex numbers may model equations recently proposed in order to gain V. CAPASSO and D. BAKSTEIN, both, MIRIAM Research be used to solve algebraic equations, as well as the insight into the dynamics of complex spatiotemporal Center, Italy geometric interpretation of complex numbers and the patterns in a wide range of physical and mechanical This book is a rigorous and self-contained introduction operations involving them. problems ... One finds a large amount of exercises and open problems that can serve as a starting point for to the theory of continuous-time stochastic processes. The theoretical parts of the book are augmented with Abalance of theory and applications, the work presents further research ... The authors have produced a well­ rich exercises and problems at various levels of difficul­ concrete examples of modeling ·real-world problems ty. Aspecial feature of the book is a selection of out­ written book, which gives a good picture of what is known about the canonical equations. " from engineering, biomathematics, industrial mathe­ standing Olympiad and other mathematical contest matics, and finance using stochastic methods. problems solved by employing the methods presented -NIEU ARCHIEF VOOR WiSKUNDE 2004/APPROX 300 PP., 10 ILLUS./HARDCOVER/$64.95 (TENT.) in the text. 2001/360 PP., 2891LLUS./HARDCOVER/S82.95 ISBN 0-8176-3234-4 ISBN 0-8176-4110-6 2004/APPROX. 340 PP., 150 ILLUS./SOFTCOVER/$49.95 (TENT.) MODELING AND SIMULATION IN SCIENCE, ENGINEERING AND ISBN 0-8176-4326-S PROGRESS IN NONLINEAR DIFFERENTIAL EQUATIONS AND THEIR TECHNOLOGY APPLICATIONS, VOL 45

CALL: 1-800-777-4643 • FAX: {201) 348-4505 E-MAIL: [email protected] • VISIT: www.birkhauser.com Please mention promotion #Y9658 when ordering. Prices are valid in the Americas only and are subject to change without notice. For price and ordering information outside the Americas, Birkhiiuser please contact Birkhauser Verlag AG by E-mail: [email protected] Boston · Basel · Berlin 4/04 Promotion #Y9658 Feature Articles 394 Quantum Game Theory Steven E. Landsburg Communication over quantum channels would allow superposition of messages, so games in which players use such channels would allow superposition of strategies. The author looks at a game theory whose strategy spaces allow such quantum moves.

403 Research Mathematicians and Mathematics Education: A Critique Anthony Ralston The author reviews the contributions of research mathematicians in K-12 mathematics over the past decade and gives his assessment of positives and negatives. Communications Commentary 392 Networks 389 Opinion Bill Casselman 390 Letters to the Editor 400 A Photographic Look at the Joint 41 7 A Mathematician's Survival Guide: Meetings, Phoenix 2004 Graduate School and Early Career 412 WHAT IS ... a Billiard? Development- A Book Review YakovSinai Reviewed by Elizabeth Theta Brown 4 14 The Great Prime Number Record Races Gunter M. Ziegler 421 2004 Steele Prizes 426 2004 Veblen Prize 428 2004 Wiener Prize 431 2004 Moore Prize 433 2004 Conant Prize 435 2004AwardforDistinguished Public Service 438 2003 Morgan Prize Notices Departments of the American Mathematical Society Advertisers' Forum ...... 402 Mathematics People ...... 440 EDITOR: Andy Magid Dawson Awarded CRM-Fields Prize, AWM Essay Contest Winners ASSOCIATE EDITORS: Announced. Susanne C. Brenner, Bill Casselman (Graphics Editor), Robert J. Daverman, Nathaniel Dean, Rick Durrett, Mathematics Opportunities ...... Susan Friedlander, Robion Kirby, Steven G. Krantz, 441 Elliott H. Lieb, Mark Saul, Karen E. Smith, Audrey New NSF Funding Opportunities, Call for Nominations for Terras, Lisa Traynor CRM-Fields Prize, AP Calculus Readers Sought, News from the SENIOR WRITER and DEPUTY EDITOR: Clay Mathematics Institute, News from the International Centre Allyn Jackson for Mathematical Sciences, Training in Biological and MANAGING EDITOR: Sandra Frost Mathematical Sciences, Mathematical Social and Behavioral CONTRIBUTING WRITER: Elaine Kehoe Sciences. PRODUCTION ASSISTANT: Muriel Toupin PRODUCTION: Marcia Almeida, Kyle Antonevich, Inside the AMS ...... 443 Stephen Moye, Lori Nero, Karen Ouellette, Donna A Summer at Voice of America, Corrections to the 2003 Annual Salter, Deborah Smith, Peter Sykes Survey of the Mathematical Sciences (First Report), Correction: ADVERTISING SALES: Anne Newcomb von Neumann Awards, Deaths of AMS Members.

SUBSCRIPTION INFORMATION: Subscription prices for Volume 51 (2004) are $40llist; $321 institutional Reference and Book List ...... 445 member; $241 individual member. (The subscription price for members is included in the annual dues.) A Mathematics Calendar ...... 455 late charge of 10% of the subscription price will be imposed upon orders received from nonmembers New Publications Offered by the AMS ...... 462 after January l of the subscription year. Add for postage: Surface delivery outside the United States and India-$20; in India-$40; expedited delivery to Classifieds ...... 471 destinations in North America- $35; elsewhere-$87. Subscriptions and orders for AMS publications should AMS Membership Forms ...... 475 be addressed to the American Mathematical Society, P.O. Box 845904, Boston, MA 02284-5904 USA. All orders must be prepaid. Meetings and Conferences Table of Contents ...... 488 ADVERTISING: Notices publishes situations wanted and classified advertising, and display advertising for publishers and academic or scientific organizations. Advertising material or questions may be faxed to 401-331-3842 (indicate "Notices advertising" on fax cover sheet). SUBMISSIONS: Articles and letters may be sent to the editor by email at noti ces@math. ou. edu, by fax at 405-325-5765, or by postal mail at Department of Mathematics, 601 Elm, PHSC 423, University of Okla­ homa, Norman, OK 73019-0001. Email is preferred. Correspondence with the managing editor may be sent to noti ces@ams. org. For more information, see the From the section "Reference and Book List". NOTICES ON THE AMS WEBSITE: Most of this publi­ AMS Secretary cation is available electronically through the AMS web­ site, the Society's resource for delivering electronic products and services. Use the URL http : I /www. ams. org/noti ces/ to access the Notices on the website. Call for Nominations for 2005 Bocher Memorial Prize, Frank Nelson Cole Prize in Number Theory, Levi L. Conant Prize, Ruth (Notices of the American Mathematical Society is published Lyttle Satter Prize, and Albert Leon Whiteman Memorial monthly except bimonthly in june/ July by the American Prize ...... 449 Mathematical Society at 201 Charles Street, Providence, RI 02904-2294 USA, GST No. 12189 2046 RT****. Periodicals postage paid at Providence, Rl, and additional mailing offices. Call for Nominations for 2005 AMS Book Prize ...... 450 POSTMASTER: Send address change notices to Notices of the American Mathematical Society, P.O. Box 6248, Providence, RI 02940·6248 USA.] Publication here of the Society's street Call for Nominations for 2004 Frank and Brennie Morgan address and the other information in brackets above is a tech­ AMS-MAA-SIAM Prize ...... 451 nical requirement of the U.S. Postal Service. Tel: 401-45 5-4000, email: noti ces@ams . org. ©Copyright 2004 by the AMS Standard Cover Sheet ...... 452 American Mathematical Society. All rights reserved. Printed in the United States of America. The paper used in this journal is acid-free and falls within the guidelines established to ensure permanence and durability. Opinions expressed in signed Notices articles are those of the authors and do not necessarily reflect opinions of the editors or policies of the American Mathematical Society. Letter from the Editor

Graphics Update My employer, the University of Oklahoma, is home to are­ requires good illustrations and computers in our age markable collection of original editions of science books. have ... made it possible for amateurs to produce their own Founded by an eminent oilman and book lover, Everette L. illustrations of professional quality. Possible, but not easy, DeGolyer, who in 1947 proposed "to give [the university] and certainly not as easy as it is to produce their own math­ a blank check to buy all of the important books in printed ematical writing with .. .TpC ..All you can hope for is that editions, from Aristotle forward, significant in the history the amount of work involved in producing an illustration of science," the university's History of Science Collections is in proportion to the intrinsic difficulty of what you contains some remarkable treasures. For example, it has want to do. And the intrinsic difficulty of producing a a first edition of Galileo, with notes by the author for a good mathematical illustration inevitably means that you second edition. Mathematical should expect to do some rarities in the collection include interesting mathematics as the first printed Euclid and the well as solve interesting first Euclid in English. The lat­ computational problems ter has the following interesting along the way. Mathemat­ feature: there are little fold-out ical illustrations are a spe­ paper models in the margins cial breed-good mathe­ of the solid geometry sections.1 matical illustration almost According to the collection's always requires mathe­ curator from the 1950s through matics in the process of the 1990s, Duane Roller, who making it." (For more in­ first showed me this Euclid vol­ troductory material from ume thirty years ago, these fig­ Casselman on mathemati­ ures were there because the cal illustration, see "Pic­ English translation was done tures and Proofs", Notices, before perspective drawing had November 2000, pp. made its way to the British Isles. 1257-66.) This is such a nice story that I We hope that having a have never checked to see if graphics editor is one way Roller's theory was correct. (Nor to ease the production of have I ever seen a modern pop-up Euclid.) mathematical illustrations for Notices articles, and Those fold-out figures in the antique Euclid volume are we invite Notices authors and potential authors to explore notable as state-of-the-art (for their time and place) math­ graphical possibilities with the graphics editor. ematical graphics. Uke all good mathematical graphics, they As readers will also note, this issue contains color show the reader things which are much easier to see than graphics. Beginning with this issue, color graphics will to visualize, in this case literally adding a dimension. be a periodic (roughly quarterly) feature of the Notices. As attentive readers of the masthead will have noted, That the use of color adds a dimension to mathematical with this issue Associate Editor Bill Casselman is now the graphics seems to be widely recognized, although the Notices graphics editor. Bill will continue with his previ­ conventions and practical economics of journal publish­ ous responsibility as covers editor and in addition will work ing often preclude the use of color in much of the archival with authors of features and communications to generate literature. We hope that the opportunity to publish color images for those articles. Bill will also contribute occasional graphics in the Notices will not only add to the value of graphics articles, like the one in this issue (pages 392-393). the Notices to our readers but also inspire Notices authors As Bill writes in the preface to his book Mathematical Il­ to use the color dimension when appropriate. lustrations:2 "To explain mathematics well often -Andy Magid

1 A modern reproduction can be found on page 16 of Edward R. Tufte, Envisioning Information, Graphics Press, Cheshire, CT, 1990. The photo with this article is from a slightly later English Euclid, now in the Thomas L. Fisher Rare Book Library, . 2 Cambridge Untversity Press, to appear, and online at http: I jwww. math.ubc.ca/-cass/graphics/manual/.

APRIL 2004 NOTICES OF THE AMS 389 Letters to the Editor The Notices invites readers to submit letters and opinion pieces Remembering Max Shiffman on topics related to mathematics. Don Spencer was a close friend and Electronic submissions are pre­ sometime collaborator of Max Shiff­ ferred (noti ces-1 etters@ man. I would like to add some of ams. org); see the masthead for his words to Peter Lax's obituary postal mail addresses. Opinion ["Max Shiffman (1914-2000)," De­ pieces are usually one printed cember 2003, page 1401]. Shiffman's page in length (about 800 words). friends and colleagues, according to Letters are normally less than Spencer, contributed financially to one page long, and shorter letters keep him at Chestnut Lodge when all are preferred. other sources of funding had dried up. He was transferred back to a Califor­ nia public institution only after the burden became too great. When Shiff­ man then convinced a jury that he Where do Notices Covers Come was mentally competent, they found From? him an apartment and a job (on a Usually Notices covers come grant). Shiffman recovered and was from the articles inside the No­ appointed to a professorship at Cali­ tices. fornia State at Hayward, but accord­ In practice, most covers are ing to Spencer some quirks remained. created specifically to relate to One was that Shiffman thought he articles that have been accepted should get royalties for his theorems. for publication. Sometimes the Another was that in the heat of argu­ cover image will come straight ment Shiffman might pull a crumpled from the article, but more often document from his pocket saying, it takes off from an idea in the ar­ "Here's a paper that proves I'm sane. ticle, even sometimes a mere hint. Show me yours." But it is possible to go the other In some ways, perhaps, Shiffman way-occasionally articles have was no more mad than most of us. At been written to go with an idea for one of Spencer's gatherings in Prince­ a cover. ton, the conversation turned to his If you have an image that you continuing collaboration with Shiff­ think would make a good cover, man while Shiffman was institution­ before submitting it you should alized. Spencer said that he could write an article to go with the work with him in his lucid times but cover. The article should be of had to recognize when he was "deep­ sufficient quality to be accepted ening", because "then you couldn't on its own. But it need not be trust his theorems." When asked long; in principle, it would be rea­ how he could tell, Spencer's list of sonable for the article to simply symptoms ended with "Talking with explain the picture. him was like dropping a stone in a The simplest criterion to keep bottomless pool- you would ask him in mind is that the picture should a question at nine and get an answer try to explain something mathe­ at noon." Silence, as every wife looked matical. If the picture is striking at her husband. enough that it could stand on its If anything mitigated the tragedy own, then the accompanying ar­ of a brilliant career devastated by ticle could explain the picture, mental illness, it is that the deep, rather than the picture illustrat­ affectionate bond between Shiffman ing the article. and his colleagues was finally broken only by their deaths. -Bill Casselman Covers/Graphics Editor -Murray Gerstenhaber ([email protected]) University of Pennsylvania [email protected] (Received December 9, 2003)

390 NOTICES OF THE AMS VOLUME 51, NUMBER 4 Mathematical Word Processing • 15\TEX Typesetting • Computer Algebra

Version 5 Sharing Your Work Is Easier

• Typeset PDF in the only software that allows you to transform lt\TEX files to PDF fully hyperlinked and with embedded graphics in over 50 formats • Export documents as RTF with editable mathematics (Microsoft Word and MathType compatible) • Share documents on the web as HTML with mathematics as MathML or graphics The Gold Standard for Mathematical Publishing Scientific WorkPlace and Scientific Word make writing, sharing, and doing mathematics easier. A click of a button allows you to typeset your documents in If.T£X. And, with Scientific WorkPlace, you can compute and plot solutions with the integrated computer algebra engine, MuPAD® 2.5.

and

cKichan SOFTWARE. IN C. Tools for Scientific Creativity since /98/ Networks Bill Casselman

Every year a consortium of mathematics organi­ distribution for small p. The mean degree of nodes zations chooses a theme for the month of April, is J1 = p(N - 1). The best-known phase transition is designated "Math Awareness Month" (see also that as J1 passes through 1, the largest connected ht tp: I /www. ams. org/ams/mam. html ). This year's component starts to grow rapidly. For J1 < 1 and theme is networks. The importance of networks in large Nit contains a very small fraction of all nodes, mathematics at the moment can be gauged by the but as J1 increases it takes up a sizeable part; for fact that the Notices has recently published two J1 > 1 the fraction f taken up is the unique root of reviews of books on the topic and will publish an f = 1 - e- llf. The sizes of the other components article by Peter Sarnak on a related topic in the near decrease dramatically as well. In the following two future. figures a sequence of randomly constructed graphs "Network" is the word everybody- physicists, is shown for p = 0.8 and 1.5. The colored nodes are biologists, sociologists, engineers-except mathe­ those in the five largest components. Also shown is maticians and a few computer scientists, uses for the empirical distribution of nodal degrees. what this minority calls "graphs", i.e., a set of nodes linked together by edges. Variants exist: the edges might be oriented or labeled, or the vertices clas­ • 22 sified. If there is a connotation to "network" as 13 opposed to "graph", it is that networks arise from real life and are concerned with relations between real objects. Important examples include meta­ bolic interactions of chemicals in living things, hard-wired connections among servers on the Internet, links between World Wide Web pages, citations of references in scientific papers, conta­ gion, and electric power grids. Networks-very, very large networks- are a routine and important part of modern life. The World Wide Web has much over 1 billion live nodes. How can we make sense of something that complex? Mean 0.8 The traditional way to analyze a graph is just to Size 400 draw it and look at it, but for large networks this is ridiculous. One has to condense the information available into a small package- in other words, to compile statistics. • 258 What does a typical graph look like? In the beau­ : : tiful theory originating mostly with Alfred Renyi and 8 ~ Pal Erdos, a random graph is obtained by starting with N nodes fixed in advance and then adding edges between random pairs with probability p. There are an astonishing number of interesting results known about such graphs, the most remarkable being that certain phase transitions occur as p increases. The degree of a node in a network is the number of edges leading to it. In the random graph, distribution of degrees is binomial, well approximated by a Poisson

-Bill Casselman is a professor of mathematics at the Univer· Mean 1.5 sity of British Columbia and the graphics editor of the Size 400 Notices. His email address is cass@math. ubc. ca.

392 NOTICES OF THE AMS VOLUME 51, NUMBER 4 increasing x, de Solla Price's distribution is reasonably approximated by a power law where the 1.0 N = 400 number of nodes of degree k is proportional to = 2500 some power 1/ k 01 with IX > 2, here 2 + 1/m. The = 00 following figure shows a degree distribution aris­ relative size ing from a sample network grown by cumulative advantage, along with the theoretical beta distrib­ 0.0 ution, all plotted on a log-log graph so as to show 0 2 3 up the power law indicated by a straight line. Mean degree 2 4 8 16 64 256 1024 The figure above shows how the largest com­ Degree (log) ponent grows as 11 increases through a sequence of values for graphs of sizes 400 and 2 5 00 constructed randomly by a computer. The graph for N = oo shows the theoretical fraction taken up by the • Actual frequency largest component for p > 1, a graph which Theoretical frequency the true fraction approximates more closely for Power law large N. The traditional theory of random graphs is math­ ematically impressive and plays an important role in estimates of the efficiency of many algorithms dealing with graphs. But its role in explaining what networks encountered in real life look like is min­ imal. Real networks do not spring out of nowhere; they grow in one way or another, and the structure they acquire depends on that growth. In particu­ lar, this causes them generally to look very differ­ ent from one of the Renyi-Erd6s random graphs. As far as I know, the first account of how real networks seem to grow dates to 1965, in a paper • by the remarkable polymath Derek de Solla Price. He was interested in the statistics of the science citation network, where directed edges lead from De Solla Price's work seems to have been long a paper to each paper it refers to. It had recently neglected, and his results were rediscovered become possible to scan efficiently through large apparently independently, by Albert Barabasi i~ amounts of data based on the Science Citation much cited recent work. Barabasi introduced the Index, and de Solla Price showed that a principle now popular term "preferential attachment" for of formation he called cumulative advantage led what de Solla Price calls cumulative advantage and the term "scale free" networks for those that sat­ to a stable degree probability distribution Pk = (1 + 1/ m)B(k + 1, 2 + 1/ m). Here Pk is the probabil­ isfy a power law of degree distribution, although ity of a node having degree k, m is the mean it seems more reasonable to apply the term only effective number of journal references in papers, to those which, like the networks of de Solla Price retain a stable degree distribution as they grow. ' and B is Legendre's beta function B(x, y) = [(x)[(y) j [(x + y). He also showed that there was Further Read ing some agreement between his theory and the nature of citations in the real world. Cumulative There is a great deal of pseudoscience as well as science in this field, which is enjoying a rapid advantage can be succinctly su~marized as growth at the moment. The most difficult problem "them what has, gets" - a paper that is frequently is to fit theory with practice. A very good reference referred to will likely get more citations than one for mathematicians is the recent article 'The struc­ referred to less often. (A warning: de Solla Price's ture and function of complex networks" by Mark paper will be sobering to a naive mathematician Newman in volume 45 of the SIAM Review (2003), who would like to think that a paper's intrinsic recommended also by Rick Durrett in his March quality plays a more important role than mere Notices review of Barabasi' s book Linked. It contains popularity.) What is perhaps surprising is that the a large and useful list of references on the topic. stable degree distribution is well approximated You can find listed there the papers of de Solla Price even for small networks growing in this way, at as well as an interesting introduction to his least away from the tail end of highly cited research on citation networks. · papers. Because B(x, y) ~ [(y) j xY for fixed y and

APRIL 2004 NOTICES OF THE AMS 393 Quantum Game Theory Steven E. Lands burg

An n-person game consists of Nash equilibrium either is or is not a good predic­ 1. n sets S; (i = 1, ... , n). tor of actual behavior depending on exactly what 2. n real-valued functions sort of behavior is being modeled. Turning now to normative criteria (criteria P; : sl X ... X Sn ~ R (i=1, ... ,n). intended to judge the desirability of outcomes), The set S; is called Player i 's strategy space. The the least controversial is the criterion of Pareto­ function P; is called Player i 's payoff function. Optimality. Given two n-tuples of strategies This formulation is general enough to model s = (SI, ... , Sn) and s' = (s1, .. . , Sn). we say that s pretty much any real-world strategic interaction: we (weakly) Pareto dominates s' if P;(s) ~ P;(s') for all take S; to be the set of actions available to i; it is easy to verify that Pareto dominance is a the character called Player i, we imagine partial order, and we say that s is Pareto optimal that each player must choose some ac­ if it is maximal for this order. tion, we imagine that those actions have In general, we have no guarantees of existence some joint consequence and that P; or uniqueness for Nash equilibria or for Pareto op­ measures Player i's assessment of the tima. When they do exist, Nash equilibria and Pareto value of that consequence. Given such a optima might or might not coincide. The single model, one can ask questions that tend to most famous example in the history of game fall into two broad categories: First, what do we theory is the Prisoner's Dilemma, one form of that think the players will do (assuming, ordinarily, which can be represented by the following matrix: they are sublimely selfish and sublimely rational)? Second, what do we think the players ought to do Player Two (according to some standard of fairness or justice or morality)? Questions of the first sort call for so­ c D lution concepts; questions of the second sort call for normative criteria. The most thoroughly studied solution concept (1) (3, 3) (0, 5) is Nash equilibrium, an outcome that results when c players maximize their own payoffs, taking other Player One players' behavior as given. More precisely, an n-tuple of strategies (SI, ... , Sn) is a Nash equilib­ D (5, 0) (1,1) rium if for every i and for every s E S;, P;(SI, ... ,S;- I,Si,Si+l···Sn) Here the rows are indexed by Player 1's strategy set ~ P;(SI, . . . ,Si- I,S,Si+lo ... ,Sn). {C, D}, the columns are indexed by Player 2's strat­ egy set {C,D}, and the (i,J) entry is (PI(i,j),Pz(i,j)). The associated story, which the Steven E. Landsburg is adjunct associate professor of reader can see enacted every Tuesday night on -economics at the University of Rochester. His email N. Y.P.D. Blue, goes as follows: Two prisoners have address is 1 andsbur@troi. cc . rochester. edu. jointly committed a crime. They are separated and

394 NOTICES OF THE AMS VOLUME 51, NUMBER 4 invited to testify against each other. Each receives and Even with probability 1 - p ". It is then natural the same menu of options: You're facing a ten­ to define new payoff functions year sentence for this crime. You can either coop­ erate with your buddy (strategy C) by refusing to Pj(p, q) = pqP;(Odd, Odd) testify against him or defect from your criminal + p(1 - q)Pi(Odd, Even) partnership (strategy D) by testifying. If you both + (1 - p)qP;(Even, Odd) cooperate (i.e., if you both stay silent), we'll have to convict you of a lesser crime, which will take + (1- p)(l- q)Pi(Even, Even). three years off each of your sentences. But if you More generally, given any game G with finite defect while your buddy cooperates, we'll take five strategy sets 5;, we define a new game G+ as fol­ years off your sentence (while he serves a full lows: Let Sj be the set of all probability distribu­ term). And if you both defect, we'll take one year tions on Si, and define off each of your sentences. As the reader can (and should) verify, the unique pt:St x ·· ·X S~~R Nash equilibrium (D, D) is also the unique out­ come that is not Pareto optimal. Rational selfish by prisoners always choose the one strategy pair that both can agree is undesirable-in the sense that (3) Pj(st •... ,s~) they would both prefer (C, C).l = J P;(sl, ... , Sn)dsi(sl) ... ds~(sn). The simplest game without a Nash equilibrium St X .. ·XSn is "odds and evens", represented by the game rna- (The restriction to games with finite strategy sets trix Player Two is so that we do not have to worry about conver­ gence issues in (3).) One proves via standard fixed 0 E point theorems that the game G+ has at least one Nash equilibrium, the key point being that each Sj, unlike the original Si, can be identified with a con­ (2) vex subset of a Euclidean space on which the Pj 0 (1, -1) (-1, 1) are continuous. Player One Thus in the case of Odds and Evens, G+ is a better model of reality than G is. I want to argue E (- 1, 1) (1, - 1) that the same thing is true more generally: If G is any game with finite strategy spaces intended to model some real-world interaction, then G+ is always a better model of that same interaction. Suppose there is a Nash equilibrium in which Here's why: In the real world, players must com­ Player 1 plays Even. Then Player 2 plays Odd, so municate their strategies either to each other or Player 1 plays Odd- contradiction; and similarly to a referee or to an interrogating detective, who with Odd and Even reversed. Thus there is no Nash then computes the payoffs. And as a practical equilibrium, and without an alternative solution matter, it is quite impossible for a referee or any­ concept we are unable to predict anything other one else to prohibit the use of mixed strategies. than paralysis on the part of both players. But of Player 1 announces "I defect!" How can the referee course anyone who has ever played this game know whether Player 1 arrived at this strategy knows what actually happens: players randomize through a legal deterministic process or an illegal their strategies, and each wins half the time. random one? One might be tempted to conclude that Nash Because there is no way to prohibit mixed equilibrium is the wrong solution concept for this strategies in practice, we might as well allow game. A better conclusion is that the mathemati­ them in the model. More generally, whenever cal structure (2) is a poor model for the real-world the real world imposes limits on referees' ability game of odds and evens. A better model would to observe and/or calculate, we should improve allow for mixed (i.e. randomized) strategies. So we the model by adjusting the strategy spaces and replace the strategy space 5; = {Odd, Even} with payoff functions accordingly. the unit interval Sj = [0, 1], using p E [0, 1] to Quantum game theory begins with the obser­ model the strategy "play Odd with probability p vation that the technology of the (near?) future is likely to dictate that much communication will 1 In fact, things are even worse than that. Each player has occur through quantum channels. For example, D as a dominant strategy, which means that D is the op­ timal play regardless of whether the other player chooses players might communicate their strategies to a ref­ C or D. This is a far more powerful reason to anticipate eree via email composed on quantum computers. an outcome of (D, D) than the mere fact that (D, D) is a Such communication creates the prospect of new Nash equilibrium. strategies whose use the referee cannot detect and

APRIL 2004 NOTICES OF THE AMS 395 therefore cannot prohibit: Instead of cooperating on his first turn and the matrix u- 1 on his second. or defecting (or randomizing between the two), a Here's why that's clever: If Player Two fails to flip, player might send a message that is some quantum then the net result of the three operations is superposition of the messages "I cooperate" and "I defect". To read the message, the referee must (4) u-1 oloU=I; destroy the superposition, along with any evidence that the superposition ever existed, which makes whereas if Player Two flips, the net result is superpositions effectively impossible to prohibit. - -./2+1 -./2 What cannot be prohibited must be ( (5) u- 1 oF o u = ~ allowed; therefore, if we want to model accurately the behavior of games in which players have access to "quantum moves", we should expand our This is great for Player One, because both (4) strategy spaces accordingly. and (5) map the state represented by H into itself One might guess that a quantum move is just (remember that any scalar multiple of H is equiv­ one more way to implement a mixed strategy, so alent to H). Thus whether or not Player Two flips that there is nothing new here for game theory. The the coin, Player One is guaranteed a win. physicist David Meyer [M] was the first to publish Meyer's example shows that quantum moves a counterexample to that guess. In Meyer's exam­ can be more powerful than mere mixed strategies, ple a single coin is passed back and forth between at least in a context where quantum moves are two blindfolded players. The coin starts out heads available to only one player. But of course it is up (call this state H). Player One has the option ei­ more natural to ask what happens if both players ther to flip the coin or to return it unflipped. Then are given access to a full set of quantum moves. Player Two (still blindfolded so he doesn't know The first example of a full-fledged quantum Player One's first move) has the same option: Flip game is due to Jens Eisert, Martin Wilkens, and or don't flip. And finally, Player One gets another Maciej Lewenstein [EWL]. Let G be a two-by-two turn. If the coin ends up in its initial state H, Player One wins. If it ends up in the opposite state T (tails game, that is, a game with two players, each of up), Player Two wins. whom has a two-point strategy space, say {C, D}. Here Player One has four strategies ("flip, flip", (The reader will have no difficulty extending this "flip, don't flip", etc.). Player Two has two strate­ construction to n-by-m games.) Each player is given gies ("flip" and "don't flip"). In any mixed strategy a coin that he returns to the referee either in its Nash equilibrium, Player Two flips with probabil­ original state (to indicate a play of C) or flipped (to ity . 5, Player One flips an even number of times with indicate a play of D). A player with access to quan­ probability .5, and each wins half the games. tum moves can act on his coin with any unitary Now suppose we treat the coin as a quantum matrix and therefore return it in a state object. Its state is an equivalence class of nonzero lXH + {3T vectors in the complex vector space spanned by H ("heads") and T ("tails"); two vectors are equivalent where lX and {3 are arbitrary complex numbers, if one is a scalar multiple of the other. A physical not both zero. (Here and in everything that follows operation on the coin corresponds to a unitary I will freely abuse notation by writing lXH + {3T operation on the state space; in particular, we can both for an element of the vector space C2 and for set things up so the operation "not flip" is repre­ the state represented by that vector.) When the sented by the identity transformation and the referee observes the coin, it will appear to be un­ operation "flip" is represented (with respect to flipped or flipped with probabilities proportional the basis {H, T}) by the unitary matrix to I £X 12 and I{31 2 . As long as the coins can be treated as independent quantum entities, then indeed all we have is a fancy way to implement a mixed strat­ egy-in other words, nothing new for game theory. Meyer's example was more interesting because both players acted on a single coin. Eisert, Wilkens, Now suppose that Player One (but not Player Two) and Lewenstein (referred to henceforth as EWL) has access to a full array of quantum moves; that make their example more interesting by assuming is, instead of simply flipping or not flipping, he can the players' coins are entangled so that there apply any unitary matrix he chooses. In particular, if he is clever, Player One will choose the matrix is a single state space for the pair of coins. Explicitly, let C2 be the two-dimensonal complex - 1- i - .j2 ) vector space spanned by symbols H and T; then the ( 2 2 u-- -./2 - 1+i state space for an entangled pair is 2 -2- (6)

396 NOTICES OF THE AMS VOLUME 51, NUMBER 4 where ~ is the equivalence relation that identifies a vector with all its nonzero scalar multiples. As before, I will write, for example, H 0 H both for so we can just as well define Player One's strategy a vector in the space C2 0 C2 and for the state it space to be SUz I { ± 1}. Moreover, the group SUz can represents. A physical operation on the first coin be identified with the group s3 of unit quaternions is represented by a two-by-two unitary matrix U via the map acting on the state space S as U 0 1 . A physical SUz- s3 operation on the second coin is represented by a two-by-two unitary matrix V acting on the state (~ ~) ~A+Bj. space as 1 0 vT. Now EWL conjure the following scenario: A pair The same analysis applies to Player Two. Thus we of coins starts in the state2 define the strategy spaces (H 0 H) + (T 0 T). (8) sf= s~ = s3 1{±1} = RP3. As before, each player is handed one of the Using language loosely, I will often identify a coins and invited to indicate a play of C by strategy with either of the two quaternions that applying the identity matrix ("leaving the coin represent it. untouched") or to indicate a play of D by applying Next we motivate definitions for the payoff the matrix functions. Start with the game G: F=(o-z. 01) Player Two

D ("flipping the coin"). As long as the players restrict c themselves to the two-point strategy space {C, D}, the pair of coins lands in one of the four states (9) c (XI. Y1) (Xz, Yz) (7a) CC = (H 0 H)+ (T 0 T), Player One (7b) CD= (H 0 T)- i(T 0 H), D (X3, Y3) (X4, Y4) (7c) DC=(H 0 T)+i(T 0 H),

(7d) DD = (H 0 H) - (T 0 T). Suppose Player One plays the quaternion p and Player Two plays the quaternion q. Write the The referee now performs an observation to product as determine which of the states (7a- d) the coins occupy and makes appropriate payoffs. (10) pq = rr1(pq) + rrz(pq)i + rr3(pq)j + rr4(pq)k If players cannot be trusted to restrict them­ where the rrex are real numbers unique up to a the two strategies C and D, then the selves to sign, because p and q are defined up to a sign. Using modeler should replace the game G mathematical the notation of (7a-d), a chase through the iso­ with a new game GQ that expands the strategy morphisms reveals that the coin is transformed spaces accordingly. Player One's strategy space from the initial state CC to a final state should consist of the operations that can be effected on the state spaceS (defined in (6)) via the rr1 (pq)CC + rrz(pq)CD + rr3(pq)DC + rr4(pq)DD. action of unitary matrices on the first variable. Let When the referee observes the coins' joint state, he Uz be the group of two-by-two unitary matrices. The of the four outcomes with proba­ matrices that fix S are the scalar matrices, which observes each form a subgroup S1 c Uz. Therefore, we define bilities Player One's strategy space to be the group Uz /S1. Prob (CC) = rr1 (pq)2 Prob (CD) = rrz (pq)2 Let SUz c Uz be the subgroup of matrices with Prob (DC) = rr3(pq)2 Prob (DD) = rr4(pq)2 : determinant one; then inclusion induces an iso­ morphism Thus we should define the payoff functions by 2 depends heavily on the assumption Everything to follow 4 that the coins start in the "maximally entangled" state (11) Pf(p , q) = 7Ti(pq)2 Xi' (H ® H) + (T ® T). If they were to start in a state of the form I C<=l s ® t, then the construction that follows would only re­ construct the classical mixed strategy game G+. [EWL] 4 studies families ofquantum games parameterized by the P~(p,q) = I 7Ti(pq)2 Yi. choice of initial state. C<=l

APRIL 2004 NOTICES OF THE AMS 397 Equations (8) and (11) define the quantum game Pt(s!, ... , s~) GQ associated to the game G of equation (9). The quantum game GQ is not at all the same as the = J Pt(Sl, ... , Sn)ds!(sl) ... ds~(sn). S1 X · ·· X Sn mixed strategy game G+. Using mixed strategies, the players can jointly effect some but not all (We identify the original strategy space St with a probability distributions over the four possible subset of sr by identifying the points with the prob­ outcomes; there is, for example, no pair of ability measure concentrated ons .) Now if we equip mixed strategies that can effect the probability RP3 with its Borel cr-algebra, then GQ acquires, quite distribution naturally, the structure of a measurable game. Thus Prob (CC) = 112 Prob (CD)= 0 equation (12) becomes a meaningful definition. The payoff functions in G' should be called Prob (DC)= 0 Prob (DD) = 112. Pj or PtQ+, but I will just call them Pt. By contrast, in the game GQ any probability dis­ In the game G' there is always at least one Nash tribution at all is realizable; in fact, more is true: equilibrium, namely (J.l, J.l) where J.1 is the uniform taking Player One's strategy as given, Player Two probability distribution on RP3 . There are usually can choose a strategy that effects any desired prob­ more interesting equilibria as well. For example, let ability distribution. (Proof: Let Player One choose us return to the Prisoner's Dilemma (1). It is easy strategy p, and let r be an arbitrary unit quaternion; to verify that the following pair of mixed strategies then Player Two can play p - 1r .) constitutes a Nash equilibrium: Thus Nash equilibria must be a great rarity in quantum games; in fact, Nash equilibria exist only J.1: Player 1 plays the quaternions 1 and k, each when there exists Oi E { 1, 2, 3, 4} that maximizes with probability 1 I 2. both Xcx and Ycx; in other words, Nash equilibria (13) exist only when there is no conflict between the v: Player 2 plays the quaternions i and j, each players. with probability 1 I 2. But nobody would ever actually play the game GQ anyway. Just as there is nothing to stop our Indeed, for any unit quaternion q =a+ bi + cj + dk, players from adopting quantum strategies, there we have is also nothing to stop them from adopting mixed 5 5 quantum strategies. So we really want to study the Pz(J.l,q) = 2a2 + 2b 2 + 2c2 + 2d2 , game so that q is an optimal response to J.1 if and only if (12) a = d = 0. Thus q = i, q = j are optimal responses; So far, we have defined the game G+ only when whence so is the strategy v. Sinlilarly with the play­ the game G has finite strategy spaces, which is cer­ ers reversed. tainly not the case for GQ. So to turn (12) into a In the Nash equilibrium (13), each player's pay­ definition, we must first give a more general defi­ off is 5/2, so (13) Pareto dominates the unique nition of G+. classical equilibrium (D, D) (where the payoffs are both 1 ). Nevertheless, (13) is still Pareto subopti­ Definition. A measurable game consists of mal, being dmninated by (C, C). More generally, we would like to classify the 1. n measure spaces (St, Vt) (i = 1, ... , n). (That Nash equilibria G' G is, St is a set and Vt is a cr-algebra on St.) in where is an arbitrary two­ by-two game. The results 2. n bounded measurable functions that follow are from the forthcmning article [L). Pt : S1 X · · · X Sn - R (i = 1, ... , n). Given a strategy J.l, we define the optimal response sets Definition. Let G be a measurable game. Then a (J.l) = 3 mixed strategy for Player i consists of a probability 01 {p E RP IP1 (p, J.l) is maximized}, measure on the space (St, Vt). Oz(J.l) = {q E RP3 1Pz(J.1, q) is maximized}. Definition. Let G be a measurable game. Then we Thus (J.l, v) is a Nash equilibrium if and only if v define a game G+ as follows: Let Sj be the set of all is supported on Oz(J.l) J.1 (v). probability measures on St and define and is supported on 01 This leads us to ask: Which subsets of RP3 can occur as Ot(J.1)? The answer is: Pt : S! X · · · X S~ - R Theorem. For any J.l, each Ot(J.l) is a projective by hyperplane in RP3 .

398 NOTICES OF THE AMS VOLUME 51, NUMBER 4 Proof. We have That cuts the set of (equivalence classes of) potential Nash equilibria way down to size. But (14) pl (p, Jl) = f pl (p, q)dtJ(q), we can cut it down much further. Call a pair of weighted frames (Jl, v) realizable if it is a Nash which, for fixed Jl, is a quadratic form in the coef­ equilibrium for some quantum game. A reason­ 3 ficients rroc(P) and hence maximized (over S ) on able first guess is that every pair (Jl, v) is realizable. the intersection of s3 with the linear subspace of But the truth is quite the opposite: the main R 4 corresponding to the maximum eigenvalue of theorem of [L) establishes highly restrictive (and that form. 01 (Jl) is the image in RP3 of that linear easily checkable) necessary conditions for realiz­ subspace. Similarly, of course, for Oz. ability. Modulo some minor technical provisos, The theorem (or more precisely, the proof of the the theorem implies that for each J1 there are that (Jl, v) is theorem) has some immediate corollaries of con­ only a small finite number of v such finite number siderable interest. realizable. In many cases, that small is zero; in the remaining cases, the relevant strate­ Corollary. Let G be the arbitrary two-by-two game gies v are easy to describe. Given a particular game, of expression (9). Then in any mixed strategy quan­ this makes the search for Nash equilibria a tractable tum Nash equilibrium, Player One earns a payoff problem. of at least (X1 +Xz +X3 +X4)/4, and Player Two One can view quantum game theory as an exercise earns a payoff of at least (Y1 + Yz + Y3 + Y4)/4. in pure mathematics: Given a game G, we create a new game G! and we study its properties. But game Proof. The quadratic form (14) has trace theory has historically been interesting primarily X1 + Xz + X3 + X4 and hence a maximum eigen­ for its applications. As with quantum computing, value of at least one fourth that amount. Similarly, the applications of quantum game theory lie in the of course, for Player Two. future.3 The immediate task is to prove theorems that we expect will be useful a generation from now. mean­ Corollary. Suppose the game (9) is zero-sum, References ing that Xoc + Yoc = 0 for all 01. Then in any mixed [EWL] ]. EISERT, M. WILKENS, and M. LEWENSTEIN, Quantum strategy quantum equilibrium, Player One earns games and quantum strategies, Phys. Rev. Lett. 83 exactly(X1 + Xz + X3 + X4)/4 and Player Two earns (1999), 3077. exactly (Y1 + Yz + Y3 + Y4)/4. [L] S. LANDSBURG, Nash equilibria in quantum games, preprint available at http: I /www . l andsbu rg . To describe Nash equilibria in general, we need com/qgt.pdf. 3 to describe probability measures on RP in general. [M] D. MEYER, Quantum strategies, Phys. Rev. Lett. 8 (1999), Of course, there are a huge number of such mea­ 1052. sures, but fortunately they fall quite naturally into large equivalence classes. In particular, we say that two mixed quantum strategies J1 and tJ' are equiv­ alent if for all mixed quantum strategies v and for all 01 E {1, 2, 3, 4}, we have r 7Toc(pq)2dtJ(p)dv(q) J RP3 x RP3

= r 7Toc(pq)2dtJ'(p)dv(q) JRP3 x RP3 where the rroc are the coordinate functions on the quaternions as in (10). That is, two strategies are equivalent if one can be substituted for the other without changing either player's payoffs in any game. It turns out that equivalence classes of strate­ gies are quite large. I proved in [L) that every mixed quantum strategy is equivalent to a strategy sup­ ported on (at most) four mutually orthogonal points (that is, four points that are the images of mutu­ S3 ). Thus an equivalence ally orthogonal points in 3 It has been suggested that quantum games might have class of measures can be identified with a "weighted immediate applications in the theory of evolution, frame" in R4 , that is, a set of mutually orthogonal assuming that genetic mutations are driven by quantum vectors, each assigned a positive weight in such a events. To my knowledge there is no evidence to support way that the weights add up to one. this admittedly intriguing speculation.

APRIL 2004 NOTICES OF THE AMS 399 2004 Joint Mathematics Meetings in Phoenix

400 NOTICES OF THE AMS VOLUME 51, N UMBER 4 The 2004 joint Mathematics Meetings were held in Phoenix, AZ, january 7- 10. Some "snapshots" of Meetings events, people, and places are shown on these pages, including: registration desk, exhibits hall, speakers, prizewinners, Employment Center, email lab, Who Wants To Be A Mathematician game, Mathematics and the Arts exhibit, receptions, and meeting areas. For more details, see photo index on page 487.

APRIL 2004 N OTICES OF THE AMS 401 ~~AMS AMERICAN MATHEMATICAL SOCIETY CAMBRIDGE UNIVERSITY PRESS

ADVERTISERS' Dedicated to the advancement of knowledge through publishing and FORUM printing. (X ~ \ 1 x n-·J TUn 1 + an L.J n=i X fi(H a,.)X ll= www.cambridge.org n=l n•. n=- an ""' n , t t=IINDAWI Hindawi Publishing Corporation 410 Park Ave., 15th Floor, #287 pmb New York, NY 10022 USA Fax l-866-446-3294 (USA, toll-free)

E de Gruyter Berlin · New York www.deGruyter.com Walter de Gruyter, Inc Hawthorne, New York 10532 Phone: +1 (914)747-0110

Index of Advertisers

AM S Bookstore ...... cover 2 Hindawi Pu blishing ...... 385 Birkhai.iser ...... 386, 474 .._-:.,._,_.a;_cKichan MacKic han Software, Inc. .. . 391 SOFTWARE, INC. Tools fo r Scientific Creativity Since 1981 A K Peters ...... 420 Bu rroughs Wellcome Fund ...... 432 www.mackichan.com Press ...... 434 Email: [email protected] . +Toll-free: 877-724-9673 +Fax: 360-394-6039 Cambridge University Press . . . 4 73 AMS Headlines To be featured in or to find out more about Advertisers' Forum, contact Anne Newcomb by and Deadlines ...... cover 3 phone at 1. 800.321.4267 (1.401.455.4084, worldwide) or send email to [email protected]. Springer ...... cover 4 14/04 1 Research Mathematicians and Mathematics Education: A Critique Anthony Ralston

Not since the New Math period of the 1960s had university mathematicians played such important roles in K-12 education as in California during the 1990s. -David Klein, "A brief history of American K-12 mathematics education in the 20th century" [1]

ince at least the publication of A Nation at those in the Third International Mathematics Study Risk [2] in 1983, there has been ferment [4], as well as scores on various tests, together about precollege mathematics education in with a plethora of anecdotal evidence, suggest that Sthe U.S. Since then, but particularly since far from achieving (the first) President Bush's aim 199 3, research mathematicians have been that U.S. mathematics education should be second more active on the precollege mathematics scene to none by 2000, mathematics education in the than at any time since the days of the New Math U.S. is still nowhere near "second to none". Thus, in the 1960s. Indeed, the pages of the Notices have the efforts of research mathematicians, working to­ regularly had articles, opinion pieces, and letters gether with the other constituencies in math edu­ on the subject of school mathematics. This seems, cation, will be needed if the current situation is to therefore, a good time to review the impact of re­ show improvement. As noted in [5], "one of the search mathematicians on school mathematics most important ways mathematicians can be so­ over the past ten years. In this article I will consider cially responsible [is] by working to improve pre­ where the intervention of research mathematicians college math education." in school mathematics has had favorable results But instead of cooperation, we have had for the and where the results have been less than favor­ past decade, although recently at a lower decibel able. level, the Math Wars [6], [7], which pit (mainly) re­ Just about everyone agrees that research math­ search mathematicians against (mainly) college ematicians have the knowledge and expertise to and university mathematics educators and school make important contributions to the improvement mathematics teachers. No matter which side, if ei­ of school mathematics in the U.S. Indeed, it has been ther, of these wars you are on, it is clear that they stated by a prominent mathematics educator that have, at least, prevented more improvement in U.S. "American mathematics education has benefited school mathematics education than might other­ from a virtually continual stream of support from wise have been achieved. Throughout this article I prominent research mathematicians" [3]. Equally, will use the terms "traditional" and "progressive" just about everyone believes that school mathe­ (or "reform") to designate the two sides in the Math matics is in great, some would say dire, need of im­ Wars, because, whether you like these terms or provement. International comparisons, such as not, they have become traditional (!) in the litera­ ture. Anthony Ralston is professor emeritus of computer science My aim here is not to refight or continue to fight and mathematics at the State University of New York at Buffalo and an academic visitor in the Department of the Math Wars, at least insofar as their mathemat­ Computing at the Imperial College of Science, Technology ical substance is concerned. However, I will not hes­ and Medicine in . His email address is itate to criticize the tactics of the math warriors [email protected]. when I think these have been counterproductive.

APRIL2004 NOTICES OF THE AMS 403 On the Plus Side difficult because of the errors, sometimes consid­ Why is there agreement among most university erable numbers of them, that have been found by mathematicians and mathematics educators that research mathematicians after publication. The er­ the potential contribution of university mathe­ rors found are often of a trivial and easily fixable maticians to school mathematics education is great? nature, but their existence must serve to weaken Is it because their knowledge of mathematics is su­ the effect of a curriculum proposal. perior to that of mathematics educators? Partly, Teaching Methods even though most mathematics educators, although On the mathedlistserv of which I am a member, they know far less mathematics than research I am often impressed by the insight of other mem­ mathematicians, are knowledgeable enough about bers, often research mathematicians, about how school mathematics to design and implement cur­ particular topics of school mathematics might be ricula for school mathematics. More important, I approached in novel and useful ways. Sometimes think, is the generally deep knowledge that uni­ the ideas may be impractical because of aspects of versity mathematicians have of mathematics, be­ school mathematics not fully understood by uni­ cause this gives them useful insight into what top­ versity mathematicians, but even when this is the ics are particularly important in school mathematics case, the ideas may suggest changes in approach and sometimes into good approaches for teaching to mathematics educators and teachers. Indeed, re­ these topics. With that said, here is a list of areas search mathematicians should be used much more where research mathematicians can make and have than they have been as a source of ideas for teach­ made useful contributions to the debate about ing the content of school mathematics. school mathematics. Another aspect of this concerns methods that Topics have become popular in school mathematics be­ The division of fractions was not included in the cause they are easy to teach but are counterpro­ original version of one of the more popular reform ductive to the understanding of the underlying curricula (Connected Mathematics Program). What­ mathematics. One such is the "infamous" [12, p. 7] ever the reason for this omission, knowledge of FOIL algorithm to multiply two linear polynomials how to divide fractions, while of limited value by bypassing the distributive law entirely. Research when the fractions consist entirely of numbers, is mathematicians have an important role to play in clearly important in much of more advanced math­ apprising mathematics educators and teachers of ematics. When this omission was noticed by a when the methods they teach, although perhaps well-known university mathematician, the devel­ narrowly effective and efficient, give the wrong opers of the curriculum in question promised to mathematical lesson and may, therefore, ill serve remedy this in the next version of the curriculum the student. and did so [8]. In-Service Education of Teachers A less clear-cut example was the postponement Although there may be doubts about whether of teaching the quadratic formula until the twelfth university mathematicians understand enough grade in another popular curriculum (Interactive about school pedagogical issues for them to con­ Mathematics Program) [9], [10]. Surely this is idio­ tribute much about these, there can be no doubt syncratic, and even if this delay is defensible, it is that they can contribute to improving the mathe­ useful to have university mathematicians question matical understanding of elementary and secondary choices of this kind. school teachers. Even if it is true that in-service In general, it is valuable for curriculum devel­ courses for teachers in summers and on weekends opers to have advice and criticism from university will probably never reach a significant fraction of mathematicians about what to include or omit, if teachers and often reach just those teachers who only because it is all too easy to make some bad are already among the most competent, these decisions when immersed in the details of a cur­ courses are an important way of getting mathe­ riculum to be used for all of elementary or all of matical knowledge to teachers that their preservice secondary school mathematics. education did not. Even if the subject matter of such Errors courses is not directly applicable to the grade Lengthy curriculum documents like books or taught by a teacher, it may nevertheless provide long papers are almost certain to contain mathe­ breadth and insight that will improve teaching im­ matical errors, no matter how competent the au­ mediately or in subsequent years. Considerable thors. This is particularly true when the documents numbers of research mathematicians, but surely not have many authors, as is almost always the case enough, have been involved in in-service courses for curriculum documents. Review of curriculum for teachers in recent years (e.g. [13, p. 535]). While documents by research mathematicians before pub­ such activities can only scratch the surface of the lication would always be salutary. Instead, the au­ problem of mathematics teaching in the U.S., they thors of documents such as the NCTM Standards are nevertheless valuable and deserving of [11] have made their lives considerably more applause.

404 NOTICES OF THE AMS VOLUME 51, NUMBER 4 Pre service Education of Teachers Among the handful of nonmathematicians were Another matter on which there appears to be al­ some eminent scientists, including several Nobel most universal agreement is that the mathematics prizewinners. (It was noted by one observer [7, education of prospective teachers of school math­ p. 201] that while the letter listed affiliations, chairs ematics is, with plenty of exceptions of course, held, and prizes won, there was no mention of any woefully inadequate. Indeed, in some elementary teaching awards.) education programs, prospective teachers take no The gist of the letter was a criticism of the De­ mathematics at all. Moreover, even when mathe­ partment of Education for designating five reform matics courses for prospective teachers are taught mathematics curricula as "exemplary" and five oth­ in mathematics departments, they are normally ers as "promising". (While of course criticism of the taken much less seriously by the instructors than designations of any of these ten programs was even the standard lower-division mathematics quite appropriate, the Department of Education courses. There will probably not be much dis­ had been mandated to make such designations by agreement with the claim [13, p. 535] that "uni­ Congress, which also chose the "exemplary" and versity mathematics departments must do a bet­ "promising" categories.) The six signers were, I as­ ter job of teaching their students" or with the sume, familiar with all ten programs, but it is nearly stronger statement that both mathematicians and certain that at most a handful of the endorsers were mathematics educators have [14, p. 127] "largely familiar with all ten and highly probable that most failed to help teachers learn the mathematics they were familiar with none. need in pre-service." A recent attempt at redress No doubt many readers of this article have at one of this situation is [15] a collaboration of mathe­ time or another signed advertisements in news­ matics educators and mathematicians. In any case, papers on strictly political matters whose text was more--preferably many more--research mathe­ distributed to you by someone well known to you. maticians need to get involved with the mathe­ In such cases, the text of what you are signing is matics education of prospective elementary and the whole story, and you are not endorsing opin­ secondary teachers. ions about documents not read nor about issues On the Minus Side you know nothing about directly. The Riley letter was quite different. Most Relations between the research mathematics of the community and the university mathematics edu­ endorsers were expressing an opinion about doc­ cation community have never been worse than uments they had not seen. No doubt they decided they were in the late 1990s, although they appear to do so because they trusted the opinions of those to be less vicious now than they were then. The fault who distributed the letter, and perhaps they were is surely not all on one side. However, my belief is influenced by the inclusion in the letter of an in­ that much of the fault--and most of the bitter­ flammatory quote written five years previously by ness that has resulted--is indeed the fault of the someone who served on the expert panel (of fifteen research mathematics community, almost entirely members) that made the exemplary and promising the pure mathematics research community. In the recommendations. Math Wars the research mathematics community No reader of the Notices would, I think, express has departed from its own high intellectual stan­ an opinion about a research paper he or she had dards for research and has displayed an arrogance not read, even if the paper was by a trusted col­ that has made things much worse than they need league and even if it was also vouched for by other have been. Of course, neither of these strictures ap­ trusted colleagues. This would not be intellectually plies to every research mathematician who has respectable. (Yes, there was a caveat in the open been involved with the Math Wars, but it applies letter to the effect, "While we do not necessarily to too many and particularly to many of those who agree with each of the criticisms of the programs have been most vocal. Note that these remarks are described above ... ," but that does not negate my not meant to imply that the positions taken by re­ point.) search mathematicians have necessarily been wrong An interesting contrast to the Riley letter was a or unwise, but just that they have often been ex­ letter from sixty-five mathematicians published in pressed in most unfortunate ways. the American Mathematical Monthly and The Math­ The Riley Letter ematics Teacher in 1962 [17] in which they ex­ If there is one single incident that epitomizes the pressed concern about the New Math ("Mathe­ Math Wars, it is the open letter to then Secretary maticians, reacting to the dominance of education of Education Richard Riley published in the Wash­ by professional educators who may have stressed ington Post on 18 November 1999 [16]. The letter pedagogy at the expense of content, may now stress was signed by 6 mathematicians and "endorsed" content at the expense of pedagogy and be equally by 219 others, almost all of whom were research ineffective"). The signers of this letter knew whereof mathematicians, among them many eminent ones. they spoke, but also their language was restrained,

APRIL 2004 NoTICES OF THE AMS 40:> as evidenced by the two appearances of "may" in more than we do about how to represent ideas so the quotation above. that they make sense to their students. Note that I express no opinion whatever about The folly of using standardized tests to assess the quality of the ten exemplary/promising pro­ the value of a curriculum is best illustrated (to me) grams. How good or bad they may be is not my by a story that Steve Willoughby tells in his book point here. It is certainly quite appropriate for The Other End of the Log [24]. When he was jointly mathematicians to involve themselves with the appointed to the Department of Mathematics and politics of mathematics education, but when they the School of Education at the University of Wis­ do so using the techniques of the average politi­ consin in Madison in 1960, he simultaneously be­ cian, we are all worse off. came head of the mathematics department at Wis­ Test Scores consin High School, the university's laboratory In late 2002 I wrote an oped piece [18] that sug­ school. After his first year as the high school de­ gested that one result of the No Child Left Behind partment head, a faculty member in the university Act [19] would be an epidemic of rising test scores department of mathematics noted to his colleagues in the U.S. and that this would mask a continuing that the Wisconsin High School scores in a "state­ decline in school math education in the U.S. In re­ wide school mathematics test had deteriorated se­ sponse, one of the most zealous of the math war­ riously from the previous year." Willoughby, who riors wrote a letter to the newspaper attacking my believed that the test was "hogwash," said he would claim but in effect supporting it, because his ar­ arrange it so that the scores in the high school "will gument consisted of giving tables of increasing be the highest in the state this year." And that was test scores in Sacramento. In a similar vein, when what happened, because Willoughby knew how to I visited another prominent math warrior a couple give the students the kind of test-taking skills that of years ago and expressed doubts about the 1999 assured much improved performance on the California Framework [20], his response was to statewide test. give me a file of rising test scores in California Research mathematicians do their reputation schools. as trenchant thinkers no good whatsoever when I am constantly amazed that research mathe­ they use sloppy standards to judge whether a given maticians place any faith whatever in the results curriculum is improving math education or not. The of standardized tests, much less make them the ar­ fact is that judging how well or how poorly a new biter of success or failure of a curriculum ("if the curriculum is faring is damnably difficult and ex­ test scores in these programs don't go up, California cept in very rare cases impossible, except over a isn't interested" [21]). True, mathematicians are considerable number of years. In particular, those not statisticians, but surely they generally know that who tout test scores as a measure of how well or experiments (i.e. standardized tests) with a plethora how badly a new curriculum is performing do a dis­ of uncontrolled variables cannot possibly yield service to the entire mathematics education en­ meaningful results. (I suppose the uncontrolled terprise. variables are pretty obvious: classroom preparation Calculators time for the test, teaching to the test generally, ac­ Probably no issue in math education has gen­ countability pressures on teachers to get them to erated as much heat and as little light over the past show results, year-to-year experience with the test two decades as that of the use of calculators in and the kind of questions asked, to name but a few. mathematics education. Research mathematicians, And there is the accumulating evidence that schools particularly those who oppose the so-called re­ in some states are manipulating their dropout rates form curricula, are generally opposed to the use of to assure that students who would get low scores calculators in elementary school and wary about on tests do not take the tests. See also [22, p. 645].) their use in secondary school (and, for that mat­ The number of states where so-called "high­ ter, in university mathematics). Still, despite the stakes" tests are being used is increasing rapidly. claim that "a clear majority [of academic mathe­ In Massachusetts the MCAS (Massachusetts Com­ maticians] oppose the new trends in math educa­ prehensive Assessment System) is a continuing tion" [25], of which the use of calculators is per­ source of controversy. It has, claims the 1998 Mass­ haps the most prominent, there is precious little achusetts Teacher of the Year, put teachers under evidence about how the university mathematics "enormous pressure" to prepare students for the community as a whole feels about this issue. MCAS with the result that "teachers are doing Mathematics educators, even those who favor the things that are developmentally inappropriate with use of calculators in schools, would generally admit students" [23]. Now I do not suppose readers of this that there is no conclusive evidence about the effect article know much more than I do about what is of using them in school mathematics. However, there "developmentally appropriate" for schoolchildren, are numerous studies that purport to show that the but perhaps you will agree that good elementary use of calculators in schools at worst does no harm and secondary school teachers are likely to know to the learning of traditional mathematics and may

406 NOTICES OF THE AMS VOLUME 51, NUMBER 4 at best enhance that learning. Only a very few stud­ adults. No longer. This skill itself is of essentially ies seem to contradict these results. Few of the stud­ no value any more. Devoting a considerable amount ies on either side are compelling, but there is no of the instruction in elementary school to pencil­ question about where the weight of the evidence and-paper arithmetic can only be justified in the lies [26]. twenty-first century by arguing its value in instill­ Then there is the large amount of anecdotal ev­ ing essential understanding of numbers themselves idence ("horror stories" [12, p. 9]) and oracular (e.g. place value) or for its value in preparing stu­ pronouncements that support the position that dents for the further study of mathematics. If you calculator use in school is likely to rot the brain. wish to argue that something like traditional in­ This evidence is not compelling at all, not just be­ struction in pencil-and-paper arithmetic is a nec­ cause anecdotal evidence seldom can be, but also essary part of elementary school mathematics and because it is almost always used to support a pre­ also that the use of calculators should be banned determined position. from elementary school classrooms, you need to There is certainly a widespread belief, which I argue that the use of calculators at all will inhibit share, that students arriving at American colleges sufficient learning of pencil-and-paper arithmetic. and universities have been steadily more poorly pre­ Or you need to argue that teachers will so misuse pared for college mathematics over a period ex­ calculators if they are present that adequate learn­ tending back now at least a quarter of a century. ing of pencil-and-paper arithmetic will not ensue. This belief is consonant with the scores of Amer­ However, although there is no plausible evi­ ican students on international comparisons and on dence that in the hands of good teachers calcula­ college entrance examinations such as the SATs. tors produce bad effects in elementary school class­ Some of this poor preparation of incoming college rooms, this seems to have had no effect whatever and university students may perhaps be attribut­ on the research mathematics community.1 Oh yes, able to their use of calculators in elementary or sec­ in the hands of poor teachers or in classrooms in ondary school. But at most a miniscule amount schools where very little learning of any subject could be from this cause, since the problem pre­ takes place, children may become totally dependent dates the time when there was any substantial use on calculators, resulting in what has been called of calculators in American schools, and even today "Computer-Assisted Mathematical Incompetence" such usage is far, far from universal. Nevertheless, [31]. But it seems to me to be fantasy to believe that calculators are the standard scapegoat for the poor banning calculators in such classrooms or such preparation of students in basic mathematics (e.g., schools will have any noticeable effect on the arith­ "beguiled by ever fancier calculators and comput­ metic and mathematical abilities of students. ers, teachers appear less and less able to produce The ex cathedra statements of research mathe­ students who are masters of these basic topics" [27, maticians about school arithmetic are particularly p. 868]). Too often in the debate about calculators, unhelpful, because even when the arguments they as in much of the debate about mathematics edu­ use are plausible in themselves, they often try to bias cation, research mathematicians are wont to use the debate in ways that can only antagonize the post hoc ergo propter hoc reasoning (e.g., "many are mathematics education community. Long division ignorant due to a miseducation which involves provides a case in point. At least since the publica­ heavy use of calculators" [28, p. 459]). tion of the Cockcroft Report in 1983 [32, It may be that the teaching of pencil-and-paper p. 114], which stated a belief "that it is not profitable arithmetic, which has been the gateway to the study for pupils to spend time practising the traditional of school mathematics for more than a century, is method of setting out long division on paper, but as important as it has ever been. But this position that they should normally use a calculator," there can be supported only if there is recognition that has been an ongoing debate about whether the tra­ the terms of the debate, although not necessarily ditional long division algorithm (LDA hereafter) the conclusions, about what is important to teach should be taught at all and if it is taught, what level in mathematics have been changed forever by cal­ of proficiency pupils should attain. culators. Thus This is an important debate, because it gets to the heart of the question of what is still important Even if everything had been fine with U.S. math education, we would have to 1 What appears to be almost visceral opposition to tech­ pay attention now to how the availabil­ nology sometimes leads otherwise reasonable people to ity of sophisticated calculational tools make ridiculous claims such as "A computer cannot teach changes what is important to teach [29, any more effectively than an oscilloscope can bring about p. 244]. world peace" [30, p. 991]. I am not an enthusiast for com­ puters in classrooms or in teaching, but this comment is Before calculators became ubiquitous and cheap, rubbish and evinces no understanding whatever for what it was easy to argue that some skill in pencil-and­ has been accomplished in computer-related teaching in the paper arithmetic was necessary for just about all past quarter century.

APRIL 2004 NOTICES OF THE AMS 407 in school mathematics in the calculator age. A va­ which to beat the proponents of reform curricula. riety of positions, from attaining traditional pro­ (See the next section.) ficiency in LDA to not teaching it at all, can be rea­ At one extreme it means anything whatever to sonably argued, but what is not reasonable is to do with any nontraditional school mathematics argue, as research mathematicians are wont to do curriculum [37]. Almost as extreme is the charge [33], that there is only one right answer, namely that nontraditional curricula tolerate wrong an­ teaching the traditional LDA. In [33] a variety of ben­ swers or at least make no attempt to correct wrong efits of teaching LDA are argued. All are defensi­ answers. This charge is of course false [14] and can­ ble, but the impression that only LDA can achieve not be laid correctly at the door of any developer these benefits is, frankly, nonsense. One example of a nontraditional curriculum. Less extreme but of this is the claimed value of LDA for converting also false is the claim that there are some (who?) proper fractions to decimals. Actually this con­ who believe that "there is always more than one cor­ version is more easily accomplished by an algorithm rect answer to a math problem" [38, p. 869]. Least equivalent to LDA that writes the quotient as extreme but probably most numerous are those A/B = .abcdef... and computes the successive dig­ who reject the notion that some math problems can its of the quotient by multiplying by 10 and sub­ have more than one correct answer. This last po­ tracting. Not only is this method easier to repre­ sition is not "fuzzy" in any sense; it is true. Here sent in algorithmic notation than LDA, but in are two examples: (1) Given two parallel lines and addition it suggests immediately the well-known al­ a segment longer than the distance between the two gorithm for converting repeating decimals to frac­ lines, marked out on one of them, draw an isosce­ tions. The importance of LDA is linked in [33], as les triangle with the segment as one side and the it often is, to the similar problem of the division third vertex on the other parallel line. (2) Discover of polynomials. But division of polynomials is in and explain all you can about numbers that can be fact a much simpler computation than LDA and is written as the sum of consecutive natural numbers; easily expressed in algorithmic notation by a sim­ e.g. 9=4+5,9=2+3+4;15=1+2+3+4+5. pler algorithm than LDA. 2 Now of course each of these could be transformed The penchant of research mathematicians for into a problem with a single right answer. But to claiming that there is one right way to teach a par­ do so would be to miss the point that for the first ticular aspect of school mathematics is virtually problem it is instructive for students to see that never correct. Another example is the reported other students have solved the problem in differ­ claim that there is "one right way to teach odd and ent ways. The second, more advanced, problem il­ even numbers" [7, p. 199]. lustrates the general lesson that mathematics is More generally, harking back to the papers of about investigation, conjecture, refutation or proof, Benezet in the 1930s [34], it has been suggested by and explanation. a prominent research mathematician that perhaps Of course, precision is crucial in mathematics, the teaching of the algorithms of arithmetic "should but an emphasis on precision does not require be postponed ... until grade 6" [35]. Indeed, a use­ that all problems have only one correct answer. This ful exercise for all mathematicians and mathe­ notion, which seems obvious to me, appears to be matics educators would be the gedanken experi­ anathema to many research mathematicians. It car­ ment of wondering what would happen if ries over all too easily to the notion that curricu­ mathematics education did not exist now and you lum should be rigidly specified ("one right cur­ had to invent it. How much pencil-and-paper arith­ riculum") and that the method of teaching that metic would there be? How much mental arith­ curriculum should also be rigidly specified. metic? How much use of calculators [36]? Arrogance Fuzzy Math and the One Right Answer Syndrome The "fuzzy math" epithet implies that the tra­ Those who advocate the reform of school math­ ditional math side of the Math Wars has not made ematics curricula are often accused by their de­ much attempt at the "civil, constructive discourse" tractors-almost always research mathematicians that Suzanne Wilson [7, pp. 216-29] thinks is the or journalists influenced by research mathemati­ way to end the Math Wars (see also [39, p. 488]). cians or parents influenced by journalists-of ad­ Indeed, if there is one reason more than any other vocating "fuzzy math". What is fuzzy math? It is why the Math Wars may be expected to rumble on itself a fuzzy concept, meaning whatever the crit­ into the future, it is because all too many "mathe­ ics of new school curricula want it to mean at a matical scientists have tended to look upon edu­ given time. Of course, even if the term bears no re­ cation professionals with doubts bordering on ill­ lation at all to any truth, it is a wonderful club with disguised contempt" [40, p. 21]. Indeed,"sarcasm and ridicule" [41] and "caricatures" [14, pp. 127-9], 2 The implicit assumption here, of course, is that the co­ while noticeable on both sides of the Math Wars, efficient arithmetic in the division of polynomials is, in vir­ appear to be much more prevalent on the traditional tually all cases of interest, easily done mentally. side espoused by many research mathematicians.

408 NOTICES OF THE AMS VOLUME 51, NUMBER 4 The arrogance of mathematicians towards math­ have no credentials whatever in school mathe­ ematics education manifests itself in various ways. matics education. Here are two: Not all research mathematicians are so arro­ 1. Research in Mathematics Education. At the ex­ gant with respect to their abilities as math educa­ tremist end are those research mathematicians tors. For example, Wilfrid Schmid has written, "We who believe that research in math education is [i.e. mathematicians] are not qualified to do their (virtually?) an oxymoron (e.g., "mathematics ed­ [i.e. mathematics educators'] work" [25]. And he ucation research as described in this book is, in quite correctly goes on, "Yet we are qualified as crit­ fact, inferior to descriptive discussion because ics (emphasis added) of reforms in math education." it is descriptive discussion without humility" Often the attitude of research mathematicians [42, p. 282]).3 This conclusion is reached by about the mathematical abilities of mathematics ed­ defining research as a word that can be used only ucators is manifested in their tendency to nitpick when applied to theorem/proof mathematics the documents, such as [11], of reform mathe­ research or the scientific method paradigm of matics curricula. Of course finding errors and com­ the physical sciences. From this perspective just municating them to the authors of these docu­ about everything done in education or the so­ ments is a valuable service, even when the errors cial sciences is not research. Even those who take are little more than typographical. But to use such a less extreme view often subscribe to the dic­ nit-picking as a club to try to discredit these doc­ tum that "teaching must be an art and not a sci­ uments and their authors is unworthy. I dare say ence" [42, p. 284]. This is very convenient, be­ that most of the readers of this article have writ­ cause if teaching is an art, then anyone's opinion ten books and/or long papers in which inevitably is equally valid ("I know what I like"). This au­ minor errors remained. But they surely did not ex­ tomatically gives the opinions of research math­ pect to be pilloried for these. The publication in ematicians about math education the same 2000 of Principles and Standards of School Mathe­ standing as those of mathematics educators. matics [4 7], the revision of the NCTM Standards, But although some research mathematicians has damped down the criticism of [11] to some ex­ have thought long and hard and effectively about tent but has by no means done away with it. mathematics education, too many are in the If research mathematicians would engage in grips of "the questionable belief that, just be­ "civil, constructive" criticism rather than, more cause mathematicians are good at mathematics, often than not, arrogant putdowns, the result of the they should also be able to contribute to the ef­ Math Wars would not be an endless battle to the fective presentation of elementary mathematics detriment of school mathematics education in the to an often unmotivated and umesponsive pub­ U.S. (Of course, many (most?) mathematicians are lic" [44, p. 180]. not arrogant, but those who doubt the prevalence 2. The Mathematics Credentials of Mathematics Ed­ of intellectual arrogance among mathematicians ucators. Many mathematics educators who have might care to look at a recent study of seventy been involved with the development of reform mathematicians in Britain [46].) mathematics programs have Ph.D.'s in mathe­ matics and thus might a priori be considered competent in school mathematics. But, no: My conclusion is that although a number of re­ "There is a distinction between math educators search mathematicians have contributed positively who are primarily interested in questions in­ to school mathematics education in recent years volving education, and mathematicians who (see, for example, [40, p. 18]), the research mathe­ know about mathematics" [45]. This is, of course, matics community has largely squandered an op­ ironic, because more often than not the research portunity to have a significant positive impact on mathematics critics of mathematics educators American mathematics education. Too many have used a "scattershot approach" that often takes the 3 If humility is needed in the Math Wars, a bit from the re­ form of "unsubstantiated claims and random anec­ search mathematics community might be in order. As dotes" [3]. Too often the result has been that when Igor Sharygin [43] has noted: "... professionals in pure they have become active in mathematics education, mathematics usually consider school mathematics as an research mathematicians have not lived up to the integral part of mathematical science .... And it is a grave high standards that they normally bring to their fallacy. Having indisputable authority and enormous own professional work. knowledge in their own field, these prominent scientists How could research mathematicians make a have frequently invaded a domain of public education in which they are not always competent. (Another false be­ more positive contribution to school mathematics lief is that it is quite enough to be a good mathematician education in the future? The most important way to solve the problems of mathematical education.)" Al­ would be for research mathematicians to see their though Sharygin was referring to Russia, this quote is role as colleagues of mathematics educators and equally applicable to the U.S. constructive critics of work in mathematics

APRIL 2004 NOTICES OF THE AMS 409 education. In addition, (almost all) research math­ [8) G. LAPPAN, Connected Mathematics: 6th Grade, Prentice ematicians, who have had little or no experience in Hall School Group, 2004. elementary or secondary classrooms, should learn [9) D. KLEIN, Letter in Notices Amer. Math. Soc. 44 (March to be rather more humble about school mathe­ 1997) 310. matics, since their experience in college and uni­ [10) D. FENDEL, Letter in Notices Amer. Math. Soc. 44 versity classrooms is seldom relevant to elemen­ (June/ July 1997), 656-657. tary and secondary education. [11) Curriculum and Evaluation Standards for School Mathematics, National Council Another way for research mathematicians to of Teachers of Mathe­ matics, Reston, VA, 1989. make a positive contribution to school mathemat­ [12) H. Wu, The mathematics education reform: What is ics education would be to help in recruiting more it and why should you care? http: I /math. berkeley. intellectually able people to teach mathematics edu/-wu/reform3. pdf. rather than throwing brickbats about curriculum [13) _ · _ , Professional development of mathematics matters. This is a crucial problem [48] that badly teachers, Notices Amer. Math. Soc. 46 (May 1999), needs to be addressed, and it is one where both 535-542. mathematicians and mathematics educators should [14)]. ROITMAN, Beyond the Math Wars, Contemporary Is­ find it easy to sing from the same hymn book. It sues in Mathematics Education (E. A. GAvosTo, S. G. is past time that the intellectual leaders in Amer­ KRANTZ, W. McCALLUM, eds.), Cambridge University ican mathematics started to make the case that Press, 1999, pp. 123-134. those we attract to elementary and secondary [15) The Mathematical Education of Teachers, Confer­ school teaching need to be the "brightest and best" ence Board of the Mathematical Sciences, Washington, [24, p. 57], as intellectually able as those attracted DC, 2001. to professions such as law and medicine and, yes, [16) An Open Letter to the United States Secretary of to the academic world. Education, Richard Riley, Advertisement in the Washington Post, November 18, 1999; http: I /www. Pending such an epiphany by research mathe­ mathematicallycorrect.com/riley. htm. maticians, the Math Wars, which once were avoid­ [17) L. V. AHLFORS et al., On the mathematics curriculum able, will continue to be part of the school mathe­ of the high school, Amer. Math. Monthly 69 (1962), matics landscape. The research mathematics 189-193; Math. Teacher 55 (1962), 191-195. community, through its hubris, has by and large [18) A. RALsTON, Next disaster in American education: Ris­ contributed-and continues to contribute-to a ing test scores, The Sacramento Bee (1 December worsening situation in school mathematics in the 2002). U.S., a situation that shows no signs of getting [19) No Child Left Behind Act of 2001, H.R. 1 of 2001. much better in the foreseeable future. The lesson [20) Mathematics Framework for California Public Schools, of the New Math has not been learned. California Department of Education, Sacramento, CA, 1999. Acknowledgment [21) R.]. MILGRAM, as quoted in '"Exemplary' texts with­ I would like to thank H. Burkhardt, L. Burton, drawn from California adoption process", by D. ]. E. Dubinsky, B. Jacob, G. Rising, and M. Saul for help­ HOFF, Education Week 20 (October 18, 2000). ful comments on an early draft of this paper. [22) A. H. ScHOENFEW, Purposes and Methods of Research in Mathematics Education, Notices A mer. Math. Soc. 4 7 References (June/ July 2000), 641-649. [23) M. GINLEY, as quoted in "Mass teachers blast state tests [1) D. KLEIN, A brief history of American K-12 mathe­ in new TV ads", by]. GEHRING, Education Week 20 (No­ matics education in the 20th century, in Mathemati­ vember 22, 2000). cal Cognition: Current Perspectives on Cognition, Learn­ [24) S. S. WILLOUGHBY, The Other End of the Log, Vantage ing and Instruction, edited by]. M. Royer, Information Press, 2002. Age Publishing, 2003. [25) W. SCHMID, New battles in the Math [2) A Nation at Risk: The Imperative for Educational Re­ Wars, The Har­ form, U.S. Department of Education, Washington, DC, vard Crimson, May 4, 2000. 1983; http: I /www. goa 11 i ne. org/Goa l %20L i ne/ [26) P. H. DUNHAM, Hand-held calculators in mathematics NatAtRisk.html#anchor791390. education: A research perspective, in Hand-Held Tech­ [3)]. KILPATRICK, Confronting reform, Amer. Math. Monthly nology in Mathematics and Science Education: A Col­ 104 (1997), 955-962. lection of Papers, http: I /mathcs. muh len berg. [4] W. H. ScHMIDT (ed.), Facing the Consequences: Using edu/Standards_2000_paper.htm. TIMSS for a Closer Look at U.S. Mathematics and Sci­ [2 7) G. ANDREWS, Views on high school mathematics edu­ ence Education, Kluwer, 1999. cation, Notices Amer. Math. Soc. 43 (August 1996), [5) N. KosuTZ, Letter in Notices Amer. Math. Soc. 44 (May 866-873. 1997), 543. [28) G. E. ANDREws, Commentary on assessment standards [6) A. JACKSON, The Math Wars: Parts I and II, Notices Amer. for school mathematics, Notices Amer. Math. Soc. 44 Math. Soc. 44 (June/ July 1997), 619-702; (August (April1997), 458-462. 1997) 817-827. [29) R. HoWE, The AMS and mathematics education: The [7) S. WILSON, California Dreaming: Reforming Mathe­ revision of the "NCTM Standards", Notices Amer. Math. matics Education, Yale University Press, 2003. Soc. 45 (February 1998), 243-247.

410 NOTICES OF THE AMS VOLUME 51, NUMBER 4 [30] S. G. KRANTz, Two reactions to The Mathematical Ed­ ucation of Teachers, Notices A mer. Math. Soc. 48 (Oc­ About the Cover tober 2001), 985-991. [31] R. H. EsCOBALES ]R., Letter in Notices A mer. Math. Soc. The Internet on Aprill, 2003 44 (May 1997), 542-543. This month's cover was contributed by Bill [32] Mathematics Counts, Her Majesty's Stationery Of­ Cheswick of the Lumeta Corporation and di­ fice, London, 1983. agrams the Internet as it was on Aprill of last [33] D. KLEIN and R.]. MILGRAM, The role of long division year. It nicely fits in with the topic of this in the K-12 curriculum, http: I /math. stanford. edu/ ftp/milgram/long-division-try-again.doc, year's theme for "Math Awareness Month", February 2000. which is networks. Color coding in the figure [34] L. P. BENEZET, The story of an experiment, ]. Nat. indicates distance from the scanning host. Assoc. Ed. 24 (November, December 1935), 241-244, Layout in the diagram was determined by an 301-303; 25 (January 1936), 7-8. algorithm that considers the graph as a phys­ [35] A. GLEASON, Delay the teaching of arithmetic, un­ ical system of springs and determines its min­ published manuscript. imal energy configuration. Thus, in a sense the [36] A. RALsTON, A zero-based mathematics curriculum: graph interprets itself. More information on What it is and how it might be used, Proc. Working the Lumeta Internet mapping project can be Group, Eighth Internat. Con( Math. Ed., Seville, 1996, found at http: I I research. l umeta. com/ pp. 1-8. [3 7] Glossary of Terms, http: I I ches/map/ and also in the paper "Mapping mathematicallycorrect.com/glossary.htm. and visualizing the Internet" by Cheswick, [38] H. Wu, Views on high school mathematics education, Hal Burch, and Steve Branigan in the pro­ Notices Amer. Math. Soc. 43 (August 1996), 866-872. ceedings of the 2000 USENIX Annual Techni­ [39] R. W. RILEY, The state of mathematics education: cal Conference. Designing graph layout is an Building a strong foundation for the 21st century, No­ interesting problem. A good place to start in tices Amer. Math. Soc. 45 (Aprill998), 487-491. the huge literature on this topic might be the [40] H. BAss, Mathematicians as educators, Notices Amer. proceedings publication "Graph Drawing", Math. Soc. 44 (January 1997), 18-21. Lecture Notes in Computer Science, Springer [41] R.]. MAHER, Letter in Notices Amer. Math. Soc. 44 (Au­ 1984. gust 1997), 789-790. [42] G. E. ANDREWS , Review of Mathematics Education Re­ -Bill Casselman search: A Guide for the Research Mathematician, Amer. ([email protected]) Math. Monthly 108 (March 2001), 281-285. [43]1. SHARYGIN, Mathematical education and society (an outlook from Russia and into Russia), unpublished manuscript. [44] H. CLEMENS, The Park City Institute: A mathemati­ cian's apology, Notices Amer. Math. Soc. 39 (March 1992), 180-181. [45] R.]. MILGRAM, Written testimony to the hearing of the House of Representatives Committee on Education and the Workforce; Subcommittee on Early Childhood, Youth and Families; and Subcommittee on Postsec­ ondary Education, Training and Lifelong Learning on "The Federal Role inK-12 Mathematics Reform", Feb­ ruary 2, 2000. [46] L. BURTON, Mathematicians As Enquirers-Learning about Learning Mathematics, Kluwer (to appear 2004). [4 7] Principles and Standards for School Mathematics, Na­ tional Council of Teachers of Mathematics, Reston, VA, 2000; http://standards . nctm.org. [48] V. TROEN AND K. C. BoLEs, Who's Teaching Your Chil­ dren? Why the Teacher Crisis Is Worse Than You Think and What Can Be Done about It, Yale University Press, 2003.

APRIL 2004 NOTICES OF THE AMS 411 W H A T s a Billiard? YakovSinai

Billiards are dynamical systems. In the simplest case, between geodesic flows and flows generated by a "billiard table" is a compact domain Q c Rd with quadratic differentials where the curvature is con­ a piecewise smooth boundary. For a large part of the centrated at isolated points. theory the class of smoothness plays no role. The The most thoroughly studied billiards are the two­ reader is invited to think about components of the dimensional billiards. Some are integrable, meaning boundary aQ as subsets of C"" -submanifolds of that the phase space M, minus some submanifolds codimension 1. The phase space M of the billiard is of smaller dimension, can be decomposed into two­ the unit tangent bundle of Q with the natural iden­ dimensional invariant tori, and the dynamics on each tification at the boundary torus is described by quasi-periodic functions. Ex­ amples of such billiards include: (1) billiards inside (1) v' = V- 2(v, n(q))n(q), q E aQ parallelograms, (2) billiards inside equilateral trian­ where n(q) is the inward-pointing unit normal vec­ gles, and (3) billiards inside ellipses. According to a tor at q. The reflection law (1) is not used at intersec­ popular conjecture, the set of integrable billiards tions of the several components of the boundary, can be fully described and is not much wider than this where it has no meaning. list. The dynamics {sq of a billiard is the uniform Billiards in general strictly convex smooth do­ motion with unit speed inside Q of a point repre­ mains have some properties of integrable billiards. senting the billiard ball and with the reflections off A curve ;y c Q is called a caustic if any tangent ray the boundary given by (1), -oo < t < oo . The group to ;y after reflection remains tangent to ;y. Caustics {St} preserves the Liouville measure dqdwq(v) play an important role, because some semiclassi­ where Wq is the uniform measure on the unit cal approximations of eigenfunctions of Laplacians sphere of tangent vectors to Q with given q. are described in terms of caustics. V. F. Lazutkin Mechanical systems with elastic collisions often has shown that the set of tangent vectors to all caus­ give rise to billiards. For example, a system of tics is a set of positive measure in the phase space, one-dimensional point particles with arbitrary accumulating near the boundary. In spirit this re­ masses moving freely between elastic collisions sult belongs to KAM-theory.]. Mather proved that can be described as a billiard system inside a sim­ if the curvature of aQ is zero at one or several plex whose dimension equals the number of par­ points and negative otherwise, the billiard has no ticles. The system of N hard balls of radius p in a caustics (remember the orientation of n(q)). volume V is reduced to the billiard in the domain If Q is a polygon whose angles are rational mul­ V x V x · · · x V \ C where C is the union of the tiples of rr, then the velocity along each trajectory N times of the billiard can take finitely many values. Fixing cylinders these values we get a vector field on a two­ dimensional surface whose trajectories can be Cij = {q

412 NOTICES OF THE AMS VOLUME 51, NUMBER 4 lETs are closely connected with billiards in poly­ gons. The theory of such billiards is now an actively studied topic in the theory of dynamical systems. Almost nothing is known if the angles of a poly­ gon are incommensurate with rr. Even the theory of billiards in triangles with two irrational angles awaits development. Much can be said if the curvature of oQ is strictly positive on some components of the boundary and is zero on the others. Such billiards are called hy­ perbolic. This condition links the theory of hyper­ A typical form of a stable manifold. bolic billiards with geodesic flows on manifolds of negative curvature, Anosov flows, and the general some time in the past. A typical form of srn is theory of hyperbolic dynamical systems. A similar given in the accompanying figure. definition can be easily given in the multidimen­ sional case. Probably the first person who noticed This existence of srn and urn is a manifestation the analogy between systems with elastic colli­ of the intrinsic instability of the dynamics. There­ sions and related billiards and geodesic flows on fore, hyperbolic billiards are among the most pop­ manifolds of negative curvature was the Soviet ular models of deterministic chaos. physicist N. S. Krylov. In the two-dimensional case The main problem related to hyperbolic billiards the simplest examples of hyperbolic billiards are is the problem of their ergodicity. There is a squares from which one or several strictly convex general argument due to E. Hopf that gives the scatterers are cut out. ergodicity if srn and urn have a property called L. Bunimovich showed that the billiard inside a "local transitivity", meaning that for any two close "stadium" is in a natural sense also a hyperbolic points x, y E M one can construct a continuous billiard. A stadium is a domain bounded by two path from x to y which consists of finitely many semicircles and two parallel straight segments. components such that each component belongs Later, Bunimovich and Donnay extended this result either to srn or to urn. For smooth systems where to a wider class of domains in which semicircles the sum of dimensions of srn and urn is 2d - 2, can be replaced by general strictly convex curves local transitivity follows directly from their general and the straight segments are allowed to be non­ properties. It is not so for billiards, because smooth parallel. The motion of a billiard ball on a table that components of srn or urn can be arbitrarily small. is the complement of a random or periodic con­ This difficulty can be overcome with the help of the figuration of strictly convex scatterers is called a Fundamental Theorem for hyperbolic billiards, Lorentz gas and is one of the most popular mod­ which has several versions. The first one says that els in nonequilibrium . in an arbitrary small neighborhood U of a typical Bunimovich stadia and similar billiards are pop­ point and an srn c U, the probability (in a natural ular models in the theory of quantum chaos, which sense) of points in srn for which the size of urn is studies the connections between eigenfunctions of not smaller than the size of the initial srn is greater Laplacians and ergodic properties of underlying clas­ sical dynamical systems. One of the reasons is their than some constant. This property is enough to simplicity and amenability to numerical methods. carry out Hopf's argument. In the hyperbolic theory of dynamical systems In the second version, based on the so-called a stable (unstable) manifold of a point x E M is Chernov-Sinai Ansatz, it is shown that local tran­ a local submanifold y 0 (t < 0), for with the property of local transitivity; therefore, this all y E y 2). In the case of hyperbolic (Y. G. Sinai, ed.), Encyclopedia of Mathematical Sciences, billiards, almost every point also has a stable (srn) vol. 100, Springer-Verlag, Berlin, 2000. and unstable (urn) manifold. This statement is a [2] Hard Ball Systems and the Lorentz Gas (D. Szasz, ed.), particular case of the general Hadamard-Perron Encyclopedia of Mathematical Sciences, vol. 101, theorem and is a relatively simple part of the Springer-Verlag, Berlin, 2000. theory. The new feature compared to the smooth [3] Selected Works of Eberhard Hopf with Commentaries situation is the appearance of cusp-type singular­ (Cathleen S. Morawetz, James B. Serrin, and Yakov G. ities on these manifolds, which are created by Sinai, eds.), Collected Works, vol. 17, Amer. Math. Soc., trajectories that were tangent to the boundary at Providence, Rl, 2002.

APRIL 2004 NoTICES oF THE AMS 413 The Great

2 3 5 7 11 13 17 19 23 29 Prime Number 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269 271 277 281 Record Races 283 293 307 311 31 3 3 17 331 337 347 349 353 359 367 373 379 383 389 397 401 409 419 421 431 43 3 439 443 449 457 461 463 467 479 487 491 499 503 509 521 523 541 Giinter M. Ziegler 547 557 563 569 571 577 587 593 599 601 607 613 617 619 631 641 643 647 653 659 661 673 677 683 691 701 709 719 727 733 739 743 751 757 761 769 773 787 797 809 811 821 823 827 829 839 853 857 859 863 877 881 883 887 907 911 919 929 937 941 947 953 967 971 977 983 991 997 1009 1013 1019 1021 1031 1033 1039 1049 1051 1061 1063 1069 1087 1091 1093 1097 1103 1109 1117 1123 1129 1151 1153 1163 1171 1181 1187 1193 1201 1213 1217 1223 1229 12 31 12 37 1249 1259 1277 1279 1283 1289 1291 1297 1301 1303 1307 1319 1321 1327 1361 1367 1373 1381 1399 1409 1423 1427 1429 1433 1439 1447 1451 1453 1459 1471 1481 1483 1487 1489 1493 1499 1511 1523 1531 1543 1549 1553 1559 1567 1571 1579 1583 1597 1601 1607 1609 1613 1619 1621 1627 1637 1657 1663 1667 1669 1693 1697 1699 1709 1721 1723 1733 1741 1747 1753 1759 1777 1783 1787 1789 1801 1811

The year 2003 ended with several prime Internet, get the GIMPS computer programs as well number records. For example, an effort as "their" numbers for testing, who have their per­ headed by ]ens Franke (Bonn Univer­ sonal computers do slave labor, and who report sity) led to the solution of the RSA-576 their results back to the project via the Internet. decoding problem: the factorization of Michael Shafer got the number n = 20996011 to a 174-digit decimal number. test whether 2n - 1 is prime. His PC "did it" with the GIMPS software, and it turned out that the We also have a new "largest known anwer is "yes!!" for this n. Mathworld reports that prime number": a Mersenne prime upon this success he performed a victory dance, number with 6320430 digits, called his wife and friends and began to celebrate. M = 220996011 - 1. The media attribute Let us recall: In honor of the French monk Marin the discovery to Michael Shafer, a chem­ Mersenne (1588-1648) the numbers of the form ical engineering student at Michigan Mn = 2n - 1 are called Mersenne prime numbers­ State University-but that is only part if they are prime. For this it is necessary (a nice ex­ of the story. ercise from elementary number theory) th at n it­ self is a prime. But this is not sufficient: n = 11 is Mersenne Numbers the first counter-example. In 1644 Mersenne The GIMPS project ("Great Internet Mersenne Prime claimed that Mn is a prime for n = 2, 3, 5, 7, 13, 17, Search", http: I /www. mersennse. org) was started 19, 31, 67, 127 and 257, but for no other prime in 1996. Its purpose is to search for larger and larger number smaller than 257 (and thus he got it wrong Mersenne prime numbers. The distributed com­ for exactly five cases). Mersenne prime numbers are puting project recruited volunteers who, via the rather rare: It is not known whether there are infi­ nitely many. Only the first 38 of them are known, Giinter M. Ziegler is professor of mathematics at the Tech­ plus only two more, including the newly discovered nical University, Berlin. His email address is M2o995011 which is now also the largest known [email protected]. This article was trans­ prime number. lated by the author from his article "Primzahi-Rekordjagd, Mitteilungen der Deutschen Mathematiker-Vereinigung It is quite remarkable that numbers with more 2003-4, S. 5-7. He acknowledges support from the DFG Re­ than six million digits can effectively b e tested for search Center FZT-86 "Mathematics in Key Technologies" primality. This is the genuine scientific (and in Berlin and from a DFG Leibniz grant. programming) achievement on which the new

414 NOTICES OF THE AMS VOLUME 51, N UMBER 4 record is based-that the number n = 20996011 that seems to slightly must be prime is only a little warm-up exercise for stretch the facts. (The con­ the new record. nection they can legiti­ mately make is that Cran­ PrimalityTests dell has worked on It has been shown only recently that there are exact implementing his method prime number tests that work in polynomial time­ for the prime number tests see the report in the May 2003 Notices, pp. 545-5 52. in Mathematica.) Indeed, This was a theoretical breakthrough, but it is not GIMPS works with a highly yet suitable for use "in practice." The GIMPS pro­ optimized assembly code. ject applies for each prime n a sequence of more They use floating point classical tests, which are nicely described at arithmetic because this is http: I lwww .mersenne. orglmath. htm: 1 more effective on Intel Pen­ In Phase I one looks for small prime divisors q tium processors, but this of 2n - 1. These have to satisfy (again a nice exer­ also means that the errors cise) q = 1 mod 2n and q = ±1 mod 8. Using a of floating point arithmetic modified "Sieve of Eratosthenes" adapted to such have to be detected and Marin Mersenne, 1 588-1648. factors, prime divisors of Mn up to approximately eliminated separately. 40000 (if any) are found. For this one can exploit the fact that divisibility tests for numbers of the Primality and Factoring form zn - 1 can be performed very effectively in Phases I and II of the GIMPS-sequence really do pro­ binary arithmetic. duce divisors in the case of a decomposable Mn, if In Phase II one then uses a special case of the they find any, but the third and decisive phase so-called "(p -I)-method" ofPollard(l974), which doesn't. In that case the answer will only be "de­ can be used to find factors of the form q = 2kn + 1, composable!" or not, without an explicit prime di­ for which q - 1 = 2kn consists of many small prime visor as a certificate. Thus a complete primality test factors, or (in an improved version) are highly de­ is performed, but no factorization is produced. composable except that one prime factor may be And there are good reasons for this: Not even a bit larger: To find q such that all prime factors in the special case of Mersenne numbers does one are smaller than B, one forms the product know an effective method for factoring. A method E := 0p