Pi Mu Epsilon Journal the Official Publication of the Honorary Mathematical Fraternity
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PI MU EPSILON JOURNAL THE OFFICIAL PUBLICATION OF THE HONORARY MATHEMATICAL FRATERNITY VOLUME 3 NUMBER 3 CONTENTS Plane Geometry Without Postulates - Leonard M. Blumenthal 101 An Appreciation of Giuseppe Peano - Hubert C. Kennedy . 107 On The Division of One Polynomial by Another - Michael C. Mackey . 114 Problem Department . 118 Book Reviews - . 122 Richard Kao, Franz E. Hohn, W. J. Poppelbaum, F. M. Woodworth, M. K. Fort, Jr., Gian-Carlo Rota, A. S. Householder, Murray S. Klamkin, Jane Robertson, Paul T. Bateman, W. A. Vezeau, Horace W. Norton Books Received for Review. 135 Operations Unlimited . 136 News and Notices . 146 Department Devoted to Chapter Activities . 153 , Initiates.. 160 FALL 19601 f .. Copyright 1960 by Pi Mu Epsilon Fraternity, Inc. PLANE GEOMETRY WITHOUT POSTULATESa LEONARD M. BLUMENTHAL Missouri Alpha, 1937 PI MU EPSILON JOURNAL Introduction. In his famous Elements, Euclid made a distinc- THE OFFICIAL PUBLICATIOJN tion, usually neglected today, between certain assumptions that OFTHEHONORARY MATHEMATICALFRATERNITY he called axioms, and others that he referred to as postulates. The former characterized those basic statements that expressed Francis Regan, Editor common notions (e.g., If equals are added to equals, the sums are equal) while the latter term was used for those assumptions that are peculiar to geometry (e.g., To describe a circle with any ASSOCIATE EDITORS center and radius). - The title of this paper is to be understood in the light of that Josephine Chanler Franz E. Hohn ancient discrimination. The basis for euclidean plane geometry Mary Cummings H. T. Kames presented here contains no postulates whatever (and consequent- M. S. Klarnkin ly is free of primitive or undefined terms), but it is not devoid of common notions. The sole such notion that will be used is that John J. Andrews, Business Manager of real number. This notion occurs in nearly all systems of postu- GENERAL OFFICERS OF THE FRATERNITY lates for geometry, usually without attention being called to its status as an axiom. Director General: J. S. Frame, Michigan State University It was recently observed that "according to modern standards Vice-Director General: R. H. Bing, University of Wisconsin of rigor, each branch of pure mathematics must be founded in Secretary-Treasurer General: R. V. Andree, University of Oklahoma one of two ways: (1) either its basic concepts must be defined in terms of the concepts of some prior branch of mathematics (in Councilors General: which case its theorems are deduced from those of the prior Angus E. Taylor, University of California, Los Angeles branch of mathematics with the aid of these definitions) or else Ivan Niven, University of Oregon (2) its basic concepts are taken as undefined and its theorems James C. Eaves, University of Kentucky are deduced from a set of postulates involving these undefined Marion K. Fort, Jr., University of Georgia term^."^ This foundation of plane geometry utilizes the first way; its basic concepts are defined in terms of that prior branch Chapter reports, books for review, problems for solution and solutions of mathematics - the real number system. to problems, and news items should be mailed directly to the special editors found in this issue under the various sections. Editorial cor- respondence, including manuscripts should be mailed to THE EDITOR three positive real numbers (the reason for selecting this nota- OF THE PI MU EPSILON JOURNAL, Department of Mathematics, St. tion for the three numbers will be made clear later) such that the Louis University, 221 North Grand Blvd., St. Louis 3, Mo. determinant PI MU EPSILON JOURNAL is published semi-annually at St. Louis University. SUBSCRIPTION PRICE: To Individual Members, $1.50 for 2 years; to Non-Members and Libraries, $2.00 for 2 years. Subscriptions, orders for back numbers and correspondence concerning subscriptions and advertising should be addressed to the PI MU EPSILON JOUR- NAL, Department of Mathematics, St. Louis University, 221 North is negative. There are many such triples of numbers; indeed, any Grand Blvd., St. Louis 3, Mo. three positive numbers, each one of which is less than the sum of the other two, may be selected. veceived by the editors June 23, 1960 seefootnotes. 102 PI MU EPSILON JOURNAL PLANE GEOMETRY WITHOUT POSTULATES Definition. A point is any ordered triple of non-negative real numbers (rpl,rp2,rp3) such that 0 1 1 1 1 D(P~,P~~P~^)= 1 0 pip; P^P~^4 p1r2 1 p2p; 0 p2e; p2r2 1 pap; p3pJ 0 par2 1 rp; rp; rp; 0 where rp, = pir, (i = 1,2,3). If r denotes the point (rp1,rp2,rp3) and s the point (spvsp2,sp3), we define r = s if and only if p,r = pis, (i = 1,2,3). This definition must be justified by showing that the equation has a unique non-negative real root. Let P denote the set of all points. Justification of distanc e definition. Since D(p ,p2,p3,p) = 0 it follows (as in the proof of Theorem 1) that D(pl,p2,p) $ 0, - Theorem 1. lfr is the point (0,rp2rp3), then rp2 = pip2 D(p ^p3,p) 5 0, and D(p2,p3,p) < 0. Now the equality sign can- and re= PIP. - - not hold in all three relations for if it did, then Proof. The determinant D(pl,p2,r) may be written in factored D(p ,p) = D(p2 ,p) = D(p3,p) = 0, and expansions of these form D(pl1p2,r) =-(P~P~+~P~+~~~)(P~P~+~Pz-~~~)(PIPZ-~P~+~PI) vanishing bordered third-order determinants yields ppl = pp2 = (-p lp2 +re2+rp1), and since rp = 0, pp3 = 0. But this is impossible, for then the determinant D(P,,P~,~)= (P~P~+~~~)~(P~P~-^)~S 0. D(p llp2,p3.p)reduces to -2.plpJ.p2p~.plp~, which does not But since D(pl,p2,p3,r) = 0, it follows that vanish. Hence we may assume D(pl,p2,p)< 0. D(P~,P~,P~).D(P~,P~,~)- crp; :I2 = 01 where crpjldenotes the minor of rpi in D(plp2,p3,r).3 This equality, together with D(p ,p2,p3) 4 0, implies that D(pl,p2,r) 2 0, and consequently D(P ,p2 ,r) = 0. Hence rp2 = pip2, and in a similar manner it is shown that 'P3 = Pip3* Denote the point (Ol~lp21~l~3)by pi, the point (P~P,~O,P~P~) by p2, and the point (p3pl,p3p2,0) by p3.(It is easily shown that Consider the function (O,P~P~,P1~3)1 etc- are points). 2. Distances. We are now in position to define the important notion - distance of two points. Definition. If P = (ppVpp2.pp3) and q = (qq vqq2,qq3) are two points of R, their distance is the unique non-negative real root of the equation D(pl,p2,p3,p,q;x) = 0, where 104 PI MU EPSILON JOURNAL PLANE GEOMETRY WITHOUT POSTULATES Its graph is a parabola whose axis is perpendicular to the Proof. Since P is a metric space, the plane contains three x-axis, and which opens upward. It crosses the x-axis at the points Pi,P$,Pi such that Pip$' = P1P2a Pfi = P2P3, Pip; = points x = (pp1 - qp1l2 and x = bpi + qp1l2. If pq2 be sub- pip3. We symbolize this by writing stituted for x in the function, the corresponding functional value Pl1P2,P3 -à Pl1P;'P; 1 d y = D(pl,p,q) 2 0, and it follows that read "p1,p2,p3 are congruent to pi,p^p;". L& p~ denote any point of E2, and consider the three non- 0 2 (pip - 5 pq2 5 (pip + pid2. Hence pq2 2 0. negative numbers pp = p*pi, pp2 = pSp;, pp3 = plpi, where plp; denotes the distance in the plane of the points p*and pi, (i = 1,2,3). Clearly D(P,,P~,P~,P)= D~~,P;,P;,P~, and since (as is well-known) the D-determinant vanishes for each quadruple of points of the plane, then D(pllp2,p3,p) = 0 and so p = (pp,,pp2,pp3) is a point of P. Hence, with each point pwf E there is associated a unique point of P. It is easily seen that each point q of P is the associate o exactly one point q*of E2. For since p ,p2,p ,q are a metri quadruple and D(pl,p2,p3,q) = 0, it follows t%at the plane 1 IP; qp; qpj x2 I contains four points contains four points Fl,T2,y3,qcongruent to that quadruple; This equation obviously has at most one non-negativ that is, root, and the above argument shows that x = pq > 0 is such a root. p1,p2,p3,q S PllP21P31~-4 Remarks. The distance of points p = (0,plp2,plp3) and p2 Then TllT2,& à PllP21~3à PisPisPi , and there is a = (P~P~,OÈP~P~is pip2; of p2 and p3 = (P~P~,P~P~,O)is p2p3; congruence of the plane with itself that carries -pll~21&into and of p and p3 is p ,p3. This explains why the notation pi,p;,p; , respectively. This congruence is unique (for since pip2, p2p3, pip3 was adopted for the three positive numbers JXP;,P~,P~)= D(p1,p2,p3) < 01 the points P~JP~~P;are not selected at the outset. They turn out to be the mutual distances collinear) and carries "q into a point qO.Consequently of the points pp2p3. The distances of a point p = (ppl,pp2,pp3) from points PlsP2sP3,q % Pi~P;*P;~l1 pl,p2,p3 are ppl,pp2,pp3, respectively. Hence we have a tri- andsoq = (qpl1qp2,qp3) = (q~pi,q~p;,qtp~);thatis,qis polar coordinate system in P.