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Topics in Geometric Group Theory. Part I
Topics in Geometric Group Theory. Part I Daniele Ettore Otera∗ and Valentin Po´enaru† April 17, 2018 Abstract This survey paper concerns mainly with some asymptotic topological properties of finitely presented discrete groups: quasi-simple filtration (qsf), geometric simple connec- tivity (gsc), topological inverse-representations, and the notion of easy groups. As we will explain, these properties are central in the theory of discrete groups seen from a topo- logical viewpoint at infinity. Also, we shall outline the main steps of the achievements obtained in the last years around the very general question whether or not all finitely presented groups satisfy these conditions. Keywords: Geometric simple connectivity (gsc), quasi-simple filtration (qsf), inverse- representations, finitely presented groups, universal covers, singularities, Whitehead manifold, handlebody decomposition, almost-convex groups, combings, easy groups. MSC Subject: 57M05; 57M10; 57N35; 57R65; 57S25; 20F65; 20F69. Contents 1 Introduction 2 1.1 Acknowledgments................................... 2 arXiv:1804.05216v1 [math.GT] 14 Apr 2018 2 Topology at infinity, universal covers and discrete groups 2 2.1 The Φ/Ψ-theoryandzippings ............................ 3 2.2 Geometric simple connectivity . 4 2.3 TheWhiteheadmanifold............................... 6 2.4 Dehn-exhaustibility and the QSF property . .... 8 2.5 OnDehn’slemma................................... 10 2.6 Presentations and REPRESENTATIONS of groups . ..... 13 2.7 Good-presentations of finitely presented groups . ........ 16 2.8 Somehistoricalremarks .............................. 17 2.9 Universalcovers................................... 18 2.10 EasyREPRESENTATIONS . 21 ∗Vilnius University, Institute of Data Science and Digital Technologies, Akademijos st. 4, LT-08663, Vilnius, Lithuania. e-mail: [email protected] - [email protected] †Professor Emeritus at Universit´eParis-Sud 11, D´epartement de Math´ematiques, Bˆatiment 425, 91405 Orsay, France. -
On Untouchable Numbers and Related Problems
ON UNTOUCHABLE NUMBERS AND RELATED PROBLEMS CARL POMERANCE AND HEE-SUNG YANG Abstract. In 1973, Erd˝os proved that a positive proportion of numbers are untouchable, that is, not of the form σ(n) n, the sum of the proper divisors of n. We investigate the analogous question where σ is− replaced with the sum-of-unitary-divisors function σ∗ (which sums divisors d of n such that (d, n/d) = 1), thus solving a problem of te Riele from 1976. We also describe a fast algorithm for enumerating untouchable numbers and their kin. 1. Introduction If f(n) is an arithmetic function with nonnegative integral values it is interesting to con- sider Vf (x), the number of integers 0 m x for which f(n) = m has a solution. That is, one might consider the distribution≤ of the≤ range of f within the nonnegative integers. For some functions f(n) this is easy, such as the function f(n) = n, where Vf (x) = x , or 2 ⌊ ⌋ f(n) = n , where Vf (x) = √x . For f(n) = ϕ(n), Euler’s ϕ-function, it was proved by ⌊ ⌋ 1+o(1) Erd˝os [Erd35] in 1935 that Vϕ(x) = x/(log x) as x . Actually, the same is true for a number of multiplicative functions f, such as f = σ,→ the ∞ sum-of-divisors function, and f = σ∗, the sum-of-unitary-divisors function, where we say d is a unitary divisor of n if d n, | d > 0, and gcd(d,n/d) = 1. In fact, a more precise estimation of Vf (x) is known in these cases; see Ford [For98]. -
Part III 3-Manifolds Lecture Notes C Sarah Rasmussen, 2019
Part III 3-manifolds Lecture Notes c Sarah Rasmussen, 2019 Contents Lecture 0 (not lectured): Preliminaries2 Lecture 1: Why not ≥ 5?9 Lecture 2: Why 3-manifolds? + Intro to Knots and Embeddings 13 Lecture 3: Link Diagrams and Alexander Polynomial Skein Relations 17 Lecture 4: Handle Decompositions from Morse critical points 20 Lecture 5: Handles as Cells; Handle-bodies and Heegard splittings 24 Lecture 6: Handle-bodies and Heegaard Diagrams 28 Lecture 7: Fundamental group presentations from handles and Heegaard Diagrams 36 Lecture 8: Alexander Polynomials from Fundamental Groups 39 Lecture 9: Fox Calculus 43 Lecture 10: Dehn presentations and Kauffman states 48 Lecture 11: Mapping tori and Mapping Class Groups 54 Lecture 12: Nielsen-Thurston Classification for Mapping class groups 58 Lecture 13: Dehn filling 61 Lecture 14: Dehn Surgery 64 Lecture 15: 3-manifolds from Dehn Surgery 68 Lecture 16: Seifert fibered spaces 69 Lecture 17: Hyperbolic 3-manifolds and Mostow Rigidity 70 Lecture 18: Dehn's Lemma and Essential/Incompressible Surfaces 71 Lecture 19: Sphere Decompositions and Knot Connected Sum 74 Lecture 20: JSJ Decomposition, Geometrization, Splice Maps, and Satellites 78 Lecture 21: Turaev torsion and Alexander polynomial of unions 81 Lecture 22: Foliations 84 Lecture 23: The Thurston Norm 88 Lecture 24: Taut foliations on Seifert fibered spaces 89 References 92 1 2 Lecture 0 (not lectured): Preliminaries 0. Notation and conventions. Notation. @X { (the manifold given by) the boundary of X, for X a manifold with boundary. th @iX { the i connected component of @X. ν(X) { a tubular (or collared) neighborhood of X in Y , for an embedding X ⊂ Y . -
Input for Carnival of Math: Number 115, October 2014
Input for Carnival of Math: Number 115, October 2014 I visited Singapore in 1996 and the people were very kind to me. So I though this might be a little payback for their kindness. Good Luck. David Brooks The “Mathematical Association of America” (http://maanumberaday.blogspot.com/2009/11/115.html ) notes that: 115 = 5 x 23. 115 = 23 x (2 + 3). 115 has a unique representation as a sum of three squares: 3 2 + 5 2 + 9 2 = 115. 115 is the smallest three-digit integer, abc , such that ( abc )/( a*b*c) is prime : 115/5 = 23. STS-115 was a space shuttle mission to the International Space Station flown by the space shuttle Atlantis on Sept. 9, 2006. The “Online Encyclopedia of Integer Sequences” (http://www.oeis.org) notes that 115 is a tridecagonal (or 13-gonal) number. Also, 115 is the number of rooted trees with 8 vertices (or nodes). If you do a search for 115 on the OEIS website you will find out that there are 7,041 integer sequences that contain the number 115. The website “Positive Integers” (http://www.positiveintegers.org/115) notes that 115 is a palindromic and repdigit number when written in base 22 (5522). The website “Number Gossip” (http://www.numbergossip.com) notes that: 115 is the smallest three-digit integer, abc, such that (abc)/(a*b*c) is prime. It also notes that 115 is a composite, deficient, lucky, odd odious and square-free number. The website “Numbers Aplenty” (http://www.numbersaplenty.com/115) notes that: It has 4 divisors, whose sum is σ = 144. -
Lecture Notes C Sarah Rasmussen, 2019
Part III 3-manifolds Lecture Notes c Sarah Rasmussen, 2019 Contents Lecture 0 (not lectured): Preliminaries2 Lecture 1: Why not ≥ 5?9 Lecture 2: Why 3-manifolds? + Introduction to knots and embeddings 13 Lecture 3: Link diagrams and Alexander polynomial skein relations 17 Lecture 4: Handle decompositions from Morse critical points 20 Lecture 5: Handles as Cells; Morse functions from handle decompositions 24 Lecture 6: Handle-bodies and Heegaard diagrams 28 Lecture 7: Fundamental group presentations from Heegaard diagrams 36 Lecture 8: Alexander polynomials from fundamental groups 39 Lecture 9: Fox calculus 43 Lecture 10: Dehn presentations and Kauffman states 48 Lecture 11: Mapping tori and Mapping Class Groups 54 Lecture 12: Nielsen-Thurston classification for mapping class groups 58 Lecture 13: Dehn filling 61 Lecture 14: Dehn surgery 64 Lecture 15: 3-manifolds from Dehn surgery 68 Lecture 16: Seifert fibered spaces 72 Lecture 17: Hyperbolic manifolds 76 Lecture 18: Embedded surface representatives 80 Lecture 19: Incompressible and essential surfaces 83 Lecture 20: Connected sum 86 Lecture 21: JSJ decomposition and geometrization 89 Lecture 22: Turaev torsion and knot decompositions 92 Lecture 23: Foliations 96 Lecture 24. Taut Foliations 98 Errata: Catalogue of errors/changes/addenda 102 References 106 1 2 Lecture 0 (not lectured): Preliminaries 0. Notation and conventions. Notation. @X { (the manifold given by) the boundary of X, for X a manifold with boundary. th @iX { the i connected component of @X. ν(X) { a tubular (or collared) neighborhood of X in Y , for an embedding X ⊂ Y . ◦ ν(X) { the interior of ν(X). This notation is somewhat redundant, but emphasises openness. -
Anti-Conformism and Social Networks Alexis Poindron
Anti-conformism and Social Networks Alexis Poindron To cite this version: Alexis Poindron. Anti-conformism and Social Networks. Economics and Finance. 2016. dumas- 02102411 HAL Id: dumas-02102411 https://dumas.ccsd.cnrs.fr/dumas-02102411 Submitted on 18 Apr 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Anti-conformism and Social Networks Alexis Poindron Université Paris I Sorbonne, UFR de sciences économiques 02 Master 2 Économie Théorique et Empirique 2015-2016 Mémoire présenté et soutenu par Alexis Poindron le 17/05/2016 Sous la direction de Michel Grabisch et Agnieszka Rusinowska May 17, 2016 L’Université de paris I Panthéon Sorbonne n’entend donner aucune approbation, ni désappro- bation aux opinions émises dans ce mémoire ; elle doivent être considérées comme propres à leur auteur. 1 Contents 1 Introduction . .3 1.1 Literature review and context of this master thesis . .3 1.2 Some notations and definitions . .5 2 Generalized OWA . .6 2.1 GOWA: definitions and exemples . .6 2.2 Discussion on GOWA . .8 3 Terminal states and classes with GOWA: homogeneous framework . .9 3.1 Preliminary results . .9 3.2 List of possible terminal classes and unicity . -
Rapid Arithmetic
RAPID ARITHMETIC QUICK AND SPE CIAL METHO DS IN ARITH METICAL CALCULATION TO GETHE R WITH A CO LLE CTION OF PUZZLES AND CURI OSITIES OF NUMBERS ’ D D . T . O CONOR SLOAN E , PH . LL . “ ’ " “ A or o Anda o Ele ri i ta da rd le i a Di ion uth f mfic f ct c ty, S n E ctr c l ct " a Eleme a a i s r n r Elec r a l C lcul on etc . etc y, tp y t ic t , , . NE W Y ORK D VA N NOS D O M N . TRAN C PA Y E mu? WAuw Sum 5 PRINTED IN THE UNITED STATES OF AMERICA which receive little c n in ne or but s a t treatment text books . If o o o f doin i e meth d g an operat on is giv n , it is considered n e ough. But it is certainly interesting to know that there e o f a t are a doz n or more methods adding, th there are a of number of ways applying the other three primary rules , and to find that it is quite within the reach of anyo ne to io add up two columns simultaneously . The multiplicat n table for some reason stops abruptly at twelve times ; it is not to a on or hard c rry it to at least towards twenty times . n e n o too Taking up the questio of xpone ts , it is not g ing far to assert that many college graduates do not under i a nd stand the meaning o a fractional exponent , as few can tell why any number great or small raised to the zero power is equa l to one when it seems a s if it ought to be equal to zero . -
Variant of a Theorem of Erdős on The
MATHEMATICS OF COMPUTATION Volume 83, Number 288, July 2014, Pages 1903–1913 S 0025-5718(2013)02775-5 Article electronically published on October 29, 2013 VARIANT OF A THEOREM OF ERDOS˝ ON THE SUM-OF-PROPER-DIVISORS FUNCTION CARL POMERANCE AND HEE-SUNG YANG Abstract. In 1973, Erd˝os proved that a positive proportion of numbers are not of the form σ(n) − n, the sum of the proper divisors of n.Weprovethe analogous result where σ is replaced with the sum-of-unitary-divisors function σ∗ (which sums divisors d of n such that (d, n/d) = 1), thus solving a problem of te Riele from 1976. We also describe a fast algorithm for enumerating numbers not in the form σ(n) − n, σ∗(n) − n,andn − ϕ(n), where ϕ is Euler’s function. 1. Introduction If f(n) is an arithmetic function with nonnegative integral values it is interesting to consider Vf (x), the number of integers 0 ≤ m ≤ x for which f(n)=m has a solution. That is, one might consider the distribution of the image of f within the nonnegative integers. For some functions f(n) this is easy, such√ as the function 2 f(n)=n,whereVf (x)=x,orf(n)=n ,whereVf (x)= x.Forf(n)= ϕ(n), Euler’s ϕ-function, it was proved by Erd˝os [Erd35] in 1935 that Vϕ(x)= x/(log x)1+o(1) as x →∞. Actually, the same is true for a number of multiplicative functions f,suchasf = σ, the sum-of-divisors function, and f = σ∗,thesum-of- unitary-divisors function, where we say d is a unitary divisor of n if d | n, d>0, and gcd(d, n/d) = 1. -
Differentiable Mappings in the Schoenflies Theorem
COMPOSITIO MATHEMATICA MARSTON MORSE Differentiable mappings in the Schoenflies theorem Compositio Mathematica, tome 14 (1959-1960), p. 83-151 <http://www.numdam.org/item?id=CM_1959-1960__14__83_0> © Foundation Compositio Mathematica, 1959-1960, tous droits réser- vés. L’accès aux archives de la revue « Compositio Mathematica » (http: //http://www.compositio.nl/) implique l’accord avec les conditions gé- nérales d’utilisation (http://www.numdam.org/conditions). Toute utili- sation commerciale ou impression systématique est constitutive d’une infraction pénale. Toute copie ou impression de ce fichier doit conte- nir la présente mention de copyright. Article numérisé dans le cadre du programme Numérisation de documents anciens mathématiques http://www.numdam.org/ Differentiable mappings in the Schoenflies theorem Marston Morse The Institute for Advanced Study Princeton, New Jersey § 0. Introduction The recent remarkable contributions of Mazur of Princeton University to the topological theory of the Schoenflies Theorem lead naturally to questions of fundamental importance in the theory of differentiable mappings. In particular, if the hypothesis of the Schoenflies Theorem is stated in terms of regular differen- tiable mappings, can the conclusion be stated in terms of regular differentiable mappings of the same class? This paper gives an answer to this question. Further reference to Mazur’s method will be made later when the appropriate technical language is available. Ref. 1. An abstract r-manifold 03A3r, r > 1, of class Cm is understood in thé usual sense except that we do not require that Er be connected. Refs. 2 and 3. For the sake of notation we shall review the defini- tion of Er and of the determination of a Cm-structure on Er’ m > 0. -
Social Networks IZA Dpno
IZA DP No. 4621 Social Networks Joan de Martí Yves Zenou December 2009 DISCUSSION PAPER SERIES Forschungsinstitut zur Zukunft der Arbeit Institute for the Study of Labor Social Networks Joan de Martí Universitat Pompeu Fabra and Barcelona GSE Yves Zenou Stockholm University, IFN and IZA Discussion Paper No. 4621 December 2009 IZA P.O. Box 7240 53072 Bonn Germany Phone: +49-228-3894-0 Fax: +49-228-3894-180 E-mail: [email protected] Any opinions expressed here are those of the author(s) and not those of IZA. Research published in this series may include views on policy, but the institute itself takes no institutional policy positions. The Institute for the Study of Labor (IZA) in Bonn is a local and virtual international research center and a place of communication between science, politics and business. IZA is an independent nonprofit organization supported by Deutsche Post Foundation. The center is associated with the University of Bonn and offers a stimulating research environment through its international network, workshops and conferences, data service, project support, research visits and doctoral program. IZA engages in (i) original and internationally competitive research in all fields of labor economics, (ii) development of policy concepts, and (iii) dissemination of research results and concepts to the interested public. IZA Discussion Papers often represent preliminary work and are circulated to encourage discussion. Citation of such a paper should account for its provisional character. A revised version may be available directly from the author. IZA Discussion Paper No. 4621 December 2009 ABSTRACT Social Networks We survey the literature on social networks by putting together the economics, sociological and physics/applied mathematics approaches, showing their similarities and differences. -
The Mathematical Sociologist
The Mathematical Sociologist Newsletter of the Mathematical Sociology Section of the American Sociological Association Spring 2003 Section Officers Chair From the newsletter editor Noah E. Friedkin, Barbara Meeker, [email protected] University of California, Santa Barbara The MathSoc Section Newsletter is [email protected] back by popular demand (sort of). After a gap of Chair Elect: about two years, I volunteered once again to David Heise, Indiana University produce the newsletter. Given the many very [email protected] competitive professional accomplishments we all Past Chair, and: Section Nominations try for, it was something of a relief to campaign Committee Chair for a job absolutely no one else wants! I intend to Patrick Doreian, University of Pittsburgh do either one or two issues a year. This is the [email protected] issue for 2002-2003, covering approximately Secretary/Treasurer – 2005 April 2002 through April 2003. Dave McFarland Lisa Troyer, University of Iowa (UCLA - [email protected]) [email protected] has agreed to post newsletters and other information on a Section web page. Council 2003 The Section has been very active, as Kenneth C. Land, Duke University shown in the minutes of the Business Meeting, [email protected] prepared by 2002 Secretary-Treasurer Joe Whitmeyer, (see below). Other activities of the Jane Sell, Texas A&M University Section during the past year and some with ASA [email protected] coming up are also described below. Section Chair Noah Friedkin has written Council - 2004 an interesting comment on the Current State of Diane H. Felmlee, Mathematical Sociology; I think most of us University of California-Davis would agree wholeheartedly with his remarks. -
The Project Gutenberg Ebook #40624: a Scrap-Book Of
The Project Gutenberg EBook of A Scrap-Book of Elementary Mathematics, by William F. White This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org Title: A Scrap-Book of Elementary Mathematics Notes, Recreations, Essays Author: William F. White Release Date: August 30, 2012 [EBook #40624] Language: English Character set encoding: ISO-8859-1 *** START OF THIS PROJECT GUTENBERG EBOOK A SCRAP-BOOK *** Produced by Andrew D. Hwang, Joshua Hutchinson, and the Online Distributed Proofreading Team at http://www.pgdp.net (This file was produced from images from the Cornell University Library: Historical Mathematics Monographs collection.) Transcriber’s Note Minor typographical corrections, presentational changes, and no- tational modernizations have been made without comment. In- ternal references to page numbers may be off by one. All changes are detailed in the LATEX source file, which may be downloaded from www.gutenberg.org/ebooks/40624. This PDF file is optimized for screen viewing, but may easily be recompiled for printing. Please consult the preamble of the LATEX source file for instructions. NUMERALS OR COUNTERS? From the Margarita Philosophica. (See page 48.) A Scrap-Book of Elementary Mathematics Notes, Recreations, Essays By William F. White, Ph.D. State Normal School, New Paltz, New York Chicago The Open Court Publishing Company London Agents Kegan Paul, Trench, Trübner & Co., Ltd. 1908 Copyright by The Open Court Publishing Co.