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i i \K25063_FM" | 2017/10/5 | 11:18 | page 7 | i i DISCRETE MATHEMATICS AND ITS APPLICATIONS Handbook of DISCRETE AND COMPUTATIONAL GEOMETRY THIRD EDITION Edited by Jacob E. Goodman Joseph O’Rourke Csaba D. Tóth i i i i i i \K25063_FM" | 2017/10/5 | 11:18 | page 8 | i i CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2018 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed on acid-free paper Version Date: 20171005 International Standard Book Number-13: 978-1-4987-1139-5 (Hardback) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http:// www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Names: Goodman, Jacob E. | O’Rourke, Joseph. | Tóth, Csaba (Csaba D.) Title: Handbook of discrete and computational geometry / [edited by] Jacob Goodman, Joseph O’Rourke, Csaba Tóth. Description: Third edition. | Boca Raton : CRC Press, 2017. | Includes bibliographical references. Identifiers: LCCN 2017017843 | ISBN 9781498711395 Subjects: LCSH: Combinatorial geometry--Handbooks, manuals, etc. | Geometry--Data processing--Handbooks, manuals, etc. Classification: LCC QA167 .H36 2017 | DDC 516/.13--dc23 LC record available at https://lccn.loc.gov/2017017843 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com i i i i i i \K25063_FM" | 2017/10/6 | 14:44 | page ix | i i TABLE OF CONTENTS Prefaces ..............................................................................xi Contributors .........................................................................xv COMBINATORIAL AND DISCRETE GEOMETRY..................................1 1 Finite point con(cid:12)gurations (J. Pach)...............................................3 2 Packing and covering (G. Fejes T(cid:19)oth).............................................27 3 Tilings (E. Harriss, D. Schattschneider, and M. Senechal).........................67 4 Helly-type theorems and geometric transversals (A. Holmsen and R. Wenger).....91 5 Pseudoline arrangements (S. Felsner and J.E. Goodman).........................125 6 Oriented matroids (J. Richter-Gebert and G.M. Ziegler)..........................159 7 Lattice points and lattice polytopes (A. Barvinok) ...............................185 8 Low-distortion embeddings of (cid:12)nite metric spaces (P. Indyk, J. Matou(cid:20)sek, and A. Sidiropoulos) ..............................211 9 Geometry and topology of polygonal linkages (R. Connelly and E.D. Demaine)...233 10 Geometric graph theory (J. Pach)................................................257 11 Euclidean Ramsey theory (R.L. Graham) ........................................281 12 Discrete aspects of stochastic geometry (R. Schneider)...........................299 13 Geometric discrepancy theory and uniform distribution (J.R. Alexander, J. Beck, and W.W.L. Chen)..............................331 14 Polyominoes (G. Barequet, S.W. Golomb, and D.A. Klarner).....................359 POLYTOPES AND POLYHEDRA..................................................381 15 Basic properties of convex polytopes (M. Henk, J. Richter-Gebert, and G.M. Ziegler)............................383 16 Subdivisions and triangulations of polytopes (C.W. Lee and F. Santos)...........415 17 Face numbers of polytopes and complexes (L.J. Billera and A. Bj(cid:127)orner)..........449 18 Symmetry of polytopes and polyhedra (E. Schulte)...............................477 19 Polytope skeletons and paths (G. Kalai) .........................................505 20 Polyhedral maps (U. Brehm and E. Schulte) .....................................533 COMBINATORIAL AND COMPUTATIONAL TOPOLOGY.......................549 21 Topological methods in discrete geometry (R.T. Z(cid:20)ivaljevi(cid:19)c).......................551 22 Random simplicial complexes (M. Kahle).........................................581 23 Computational topology of graphs on surfaces (E. Colin de Verdi(cid:18)ere).............605 24 Persistent homology (H. Edelsbrunner and D. Morozov)..........................637 25 High-dimensional topological data analysis (F. Chazal)...........................663 ALGORITHMS AND COMPLEXITY OF FUNDAMENTAL GEOMETRIC OBJECTS......................................685 26 Convex hull computations (R. Seidel)............................................687 27 Voronoi diagrams and Delaunay triangulations (S. Fortune)......................705 28 Arrangements (D. Halperin and M. Sharir) ...................................... 723 29 Triangulations and mesh generation (M. Bern, J. Shewchuk, and N. Amenta) ....763 30 Polygons (J. O’Rourke, S. Suri, and C.D. T(cid:19)oth) .................................787 31 Shortest paths and networks (J.S.B. Mitchell)....................................811 32 Proximity algorithms (J.S.B. Mitchell and W. Mulzer)...........................849 33 Visibility (J. O’Rourke)..........................................................875 34 Geometric reconstruction problems (Y. Disser and S.S. Skiena)..................897 i i i i i i \K25063_FM" | 2017/10/6 | 14:44 | page x | i i x 35 Curve and surface reconstruction (T.K. Dey).....................................915 36 Computational convexity (P. Gritzmann and V. Klee)............................937 37 Computational and quantitative real algebraic geometry (S. Basu and B. Mishra)....................................................969 GEOMETRIC DATA STRUCTURES AND SEARCHING..........................1003 38 Point location (J. Snoeyink).....................................................1005 39 Collision and proximity queries (Y. Kim, M.C. Lin, and D. Manocha)...........1029 40 Range searching (P.K. Agarwal).................................................1057 41 Ray shooting and lines in space (M. Pellegrini)..................................1093 42 Geometric intersection (D.M. Mount) ...........................................1113 43 Nearest neighbors in high-dimensional spaces (A. Andoni and P. Indyk).........1135 COMPUTATIONAL TECHNIQUES............................................... 1157 44 Randomizaton and derandomization (O. Cheong, K. Mulmuley, and E. Ramos).................................1159 45 Robust geometric computation (V. Sharma and C.K. Yap)......................1189 46 Parallel algorithms in geometry (M.T. Goodrich and N. Sitchinava) .............1225 47 Epsilon-approximations and epsilon-nets (N. Mustafa and K. Varadarajan)......1241 48 Coresets and sketches (J. Phillips) .............................................. 1269 APPLICATIONS OF DISCRETE AND COMPUTATIONAL GEOMETRY........1289 49 Linear programming (M. Dyer, B. G(cid:127)artner, N. Megiddo, and E. Welzl)..........1291 50 Algorithmic motion planning (D. Halperin, O. Salzman, and M. Sharir).........1311 51 Robotics (D. Halperin, L.E. Kavraki, and K. Solovey)...........................1343 52 Computer graphics (D. Dobkin and S. Teller)...................................1377 53 Modeling motion (L.J. Guibas and M. Roeloffzen)...............................1401 54 Pattern recognition (J. O’Rourke and G.T. Toussaint) ..........................1421 55 Graph drawing (E. Di Giacomo, G. Liotta, and R. Tamassia)...................1451 56 Splines and geometric modeling (C.L. Bajaj)....................................1479 57 Solid modeling (C.M. Hoffmann and V. Shapiro)................................1503 58 Computation of robust statistics: Depth, median, and related measures (P.J. Rousseeuw and M. Hubert)...........................................1541 59 Geographic information systems (M. van Kreveld)...............................1555 60 Geometric applications of the Grassmann-Cayley algebra (N.L. White)..........1581 61 Rigidity and scene analysis (B. Schulze and W. Whiteley).......................1593 62 Rigidity of symmetric frameworks (B. Schulze and W. Whiteley) ................1633 63 Global rigidity (T. Jorda(cid:19)n and W. Whiteley)....................................1661 64 Crystals, periodic and aperiodic (M. Senechal) ..................................1695 65 Applications to structural molecular biology (H. Edelsbrunner and P. Koehl)....1709 66 Geometry and topology of genomic (A.J. Blumberg and R. Rabad(cid:19)an)............1737 GEOMETRIC SOFTWARE........................................................1777 67 Software (M. Joswig and B. Lorenz).............................................1779 68 Two computational geometry libraries: LEDA and CGAL (M. Hoffmann, L. Kettner, and S. N(cid:127)aher) .................................1799 Index of cited authors..............................................................1833 Index of de(cid:12)ned terms..............................................................1885 i i i i i i \K25063_FM" | 2017/9/15 | 19:22 | page xi | i i PREFACE TO THE FIRST EDITION While books and journals of high quality have proliferated in discrete and compu- tational geometry during recent years, there has been to date no single reference work fully accessible to the nonspecialist as well as to the specialist, covering all the major aspects of both (cid:12)elds. The Handbook of Discrete and Computational Geometry is intended to do exactly that: to make the most important results and methods in these areas of geometry readily accessible to those who use them in their everyday work, both in the academic world|as researchers in mathematics and computer science|and in the professional world|as practitioners in (cid:12)elds as diverse as operations research, molecular biology, and robotics. A signi(cid:12)cant part of the growth that discrete mathematics as a whole has experienced in recent years has consisted of a substantial development in discrete geometry. This has been fueled partly by the advent of powerful computers and by the recent explosion of activity in the relatively young (cid:12)eld of computational geometry. This synthesis between discrete and computational geometry, in which the methods and insights of each (cid:12)eld have stimulated new understanding of the other, lies at the heart of this Handbook. The phrase \discrete geometry," which at one time stood mainly for the areas of packing, covering, and tiling, has gradually grown to include in addition such areas as combinatorial geometry, convex polytopes, and arrangements of points, lines, planes, circles, andothergeometricobjectsintheplaneandinhigherdimen- sions. Similarly, \computational geometry," which referred not long ago to simply the design and analysis of geometric algorithms, has in recent years broadened its scope,andnowmeansthestudyofgeometricproblemsfromacomputationalpoint ofview,includingalsocomputationalconvexity,computationaltopology,andques- tions involving the combinatorial complexity of arrangements and polyhedra. It is clear from this that there is now a signi(cid:12)cant overlap between these two (cid:12)elds, and in fact this overlap has become one of practice as well, as mathematicians and computerscientistshavefoundthemselvesworkingonthesamegeometricproblems and have forged successful collaborations as a result. At the same time, a growing list of areas in which the results of this work are applicable has been developing. It includes areas as widely divergent as engineer- ing, crystallography, computer-aided design, manufacturing, operations research, geographic information systems, robotics, error-correcting codes, tomography, geo- metric modeling, computer graphics, combinatorial optimization, computer vision, pattern recognition, and solid modeling. Withthisinmind,ithasbecomeclearthatahandbookencompassingthemost important results of discrete and computational geometry would bene(cid:12)t not only the workers in these two (cid:12)elds, or in related areas such as combinatorics, graph theory, geometric probability, and real algebraic geometry, but also the users of this body of results, both industrial and academic. This Handbook is designed to (cid:12)ll that role. We believe it will prove an indispensable working tool both for researchers in geometry and geometric computing and for professionals who use geometric tools in their work. The Handbook covers a broad range of topics in both discrete and computa- tional geometry, as well as in a number of applied areas. These include geometric datastructures,polytopesandpolyhedra,convexhullandtriangulationalgorithms, packing and covering, Voronoi diagrams, combinatorial geometric questions, com- i i i i i i \K25063_FM" | 2017/9/15 | 19:22 | page xii | i i xii J.E. Goodman, J. O’Rourke, and C.D. T(cid:19)oth putational convexity, shortest paths and networks, computational real algebraic geometry, geometric arrangements and their complexity, geometric reconstruction problems, randomization and de-randomization techniques, ray shooting, parallel computation in geometry, oriented matroids, computational topology, mathemat- ical programming, motion planning, sphere packing, computer graphics, robotics, crystallography, and many others. A (cid:12)nal chapter is devoted to a list of available software. Results are presented in the form of theorems, algorithms, and tables, witheverytechnicaltermcarefullyde(cid:12)nedinaglossarythatprecedesthesectionin which the term is (cid:12)rst used. There are numerous examples and (cid:12)gures to illustrate the ideas discussed, as well as a large number of unsolved problems. The main body of the volume is divided into six parts. The (cid:12)rst two, on combinatorial and discrete geometry and on polytopes and polyhedra, deal with fundamental geometric objects such as planar arrangements, lattices, and convex polytopes. The next section, on algorithms and geometric complexity, discusses these basic geometric objects from a computational point of view. The fourth and (cid:12)fth sections, on data structures and computational techniques, discuss various computational methods that cut across the spectrum of geometric objects, such as randomization and de-randomization, and parallel algorithms in geometry, as well as efficient data structures for searching and for point location. The sixth section, which is the longest in the volume, contains chapters on fourteen applica- tionsareasofbothdiscreteandcomputationalgeometry,includinglow-dimensional linear programming, combinatorial optimization, motion planning, robotics, com- puter graphics, pattern recognition, graph drawing, splines, manufacturing, solid modeling, rigidity of frameworks, scene analysis, error-correcting codes, and crys- tallography. Itconcludeswitha(cid:12)fteenthchapter,anup-to-the-minutecompilation of available software relating to the various areas covered in the volume. A com- prehensive index follows, which includes proper names as well as all of the terms de(cid:12)ned in the main body of the Handbook. A word about references. Because it would have been prohibitive to provide complete references to all of the many thousands of results included in the Hand- book,wehavetoalargeextentrestrictedourselvestoreferencesforeitherthemost important results, or for those too recent to have been included in earlier survey booksorarticles;fortherestwehaveprovidedannotatedreferencestoeasilyacces- siblesurveysoftheindividualsubjectscoveredintheHandbook, whichthemselves contain extensive bibliographies. In this way, the reader who wishes to pursue an older result to its source will be able to do so. On behalf of the sixty-one contributors and ourselves, we would like to express our appreciation to all those whose comments were of great value to the authors of thevariouschapters: PankajK.Agarwal,BorisAronov,NogaAlon,SaugataBasu, Margaret Bayer, Louis Billera, Martin Blu(cid:127)mlinger, Ju(cid:127)rgen Bokowski, B.F. Cavi- ness,BernardChazelle,DannyChen,XiangpingChen,Yi-JenChiang,EdmundM. Clarke, Kenneth Clarkson, Robert Connelly, Henry Crapo, Isabel Cruz, Mark de Berg, Jesu(cid:19)s de Loera, Giuseppe Di Battista, Michael Drmota, Peter Eades, Ju(cid:127)rgen Eckhoff, Noam D. Elkies, Eva Maria Feichtner, Ioannis Fudos, Branko Gru(cid:127)nbaum, Dan Halperin, Eszter Hargittai, Ulli Hund, Ju(cid:127)rg Hu(cid:127)sler, Peter Johansson, Norman Johnson,AmyJosefczyk,GilKalai,GyulaK(cid:19)arolyi,KevinKlenk,W lodzimierzKu- perberg, Endre Makai, Jr., Ji(cid:20)r(cid:19)(cid:16) Matou(cid:20)sek, Peter McMullen, Hans Melissen, Bengt Nilsson, Michel Pocchiola, Richard Pollack, J(cid:127)org Rambau, Ju(cid:127)rgen Richter-Gebert, Allen D. Rogers, Marie-Fran(cid:24)coise Roy, Egon Schulte, Dana Scott, Ju(cid:127)rgen Sellen, MichaSharir,PeterShor,MaximMichailovichSkriganov,NeilJ.A.Sloane,Richard i i i i i i \K25063_FM" | 2017/9/15 | 19:22 | page xiii | i i Preface xiii P. Stanley, G(cid:19)eza T(cid:19)oth, Ioannis Tollis, Laureen Treacy, Alexander Vardy, Gert Veg- ter, Pamela Vermeer, Sini(cid:20)sa Vre(cid:19)cica, Kevin Weiler, Asia Ivi(cid:19)c Weiss, Neil White, Chee-Keng Yap, and Gu(cid:127)nter M. Ziegler. In addition, we would like to convey our thanks to the editors of CRC Press forhavingthevisiontocommissionthisHandbookaspartoftheirDiscrete Mathe- matics and Its Applications series; totheCRCstaff, fortheirhelpwiththevarious stages of the project; and in particular to Nora Konopka, with whom we found it a pleasure to work from the inception of the volume. Finally, we want to express our sincere gratitude to our families: Josy, Rachel, and Naomi Goodman, and Marylynn Salmon and Nell and Russell O’Rourke, for their patience and forbearance while we were in the throes of this project. Jacob E. Goodman Joseph O’Rourke PREFACE TO THE SECOND EDITION ThissecondeditionoftheHandbookofDiscreteandComputationalGeometryrep- resentsasubstantialrevisionofthe(cid:12)rstedition, publishedsevenyearsearlier. The new edition has added over 500 pages, a growth by more than 50%. Each chapter hasbeenthoroughlyrevisedandupdated,andwehaveaddedthirteennewchapters. The additional room permitted the expansion of the curtailed bibliographies of the (cid:12)rst edition, which often required citing other surveys to locate original sources. The new bibliographies make the chapters, in so far as is possible, self-contained. Most chapters have been revised by their original authors, but in a few cases new authors have joined the effort. All together, taking into account the chapters new to this edition, the number of authors has grown from sixty-three to eighty-four. In the (cid:12)rst edition there was one index; now there are two: in addition to the Index of De(cid:12)ned Terms there is also an Index of Cited Authors, which includes everyone referred to by name in either the text or the bibliography of each chap- ter. The (cid:12)rst edition chapter on computational geometry software has been split into two chapters: one on the libraries LEDA and CGAL, the other on additional software. There are (cid:12)ve new chapters in the applications section: on algorithms for modeling motion, on surface simpli(cid:12)cation and 3D-geometry compression, on statisticalapplications,onGeographicInformationSystemsandcomputationalcar- tography, and on biological applications of computational topology. There are new chaptersoncollisiondetectionandonnearestneighborsinhigh-dimensionalspaces. Wehaveaddedmaterialonmeshgeneration,aswellasanewchapteroncurveand surface reconstruction, and new chapters on embeddings of (cid:12)nite metric spaces, on polygonal linkages, and on geometric graph theory. Allofthesenewchapters,togetherwiththemanynewresultscontainedwithin the Handbook as a whole, attest to the rapid growth in the (cid:12)eld since preparation forthe(cid:12)rsteditionbeganadecadeago. Andasbefore,wehaveengagedtheworld’s leading experts in each area as our authors. Inadditiontothemanypeoplewhohelpedwiththepreparationofthevarious chapters comprising the (cid:12)rst edition, many of whom once again gave invaluable assistance with the present edition, we would also like to thank the following on behalf of both the authors and ourselves: Pankaj Agarwal, David Avis, Michael Baake, David Bremner, Herv(cid:19)e Br(cid:127)onnimann, Christian Buchta, Sergio Cabello, i i i i i i \K25063_FM" | 2017/9/15 | 19:22 | page xiv | i i xiv J.E. Goodman, J. O’Rourke, and C.D. T(cid:19)oth Yi-Jen Chiang, Mirela Damian, Douglas Dunham, Stefan Felsner, Lukas Finschi, Bernd G(cid:127)artner, Ewgenij Gawrilow, Daniel Hug, Ekkehard K(cid:127)ohler, Jeffrey C. La- garias, Vladimir I. Levenshtein, Casey Mann, Matthias Mu(cid:127)ller-Hannemann, Rom Pinchasi, Marc E. Pfetsch, Charles Radin, Jorge L. Ram(cid:19)(cid:16)rez Alfons(cid:19)(cid:16)n, Matthias Reitzner, Ju(cid:127)rgen Sellen, Thilo Schr(cid:127)oder, Jack Snoeyink, Hellmuth Stachel, Pavel Valtr, Nikolaus Witte, and Chee Yap. We would also like to express our appreciation to Bob Stern, CRC’s Executive Editor,whogaveusessentiallyafreehandinchoosinghowbestto(cid:12)lltheadditional 500 pages that were allotted to us for this new edition. Jacob E. Goodman Joseph O’Rourke PREFACE TO THE THIRD EDITION This third edition of the Handbook of Discrete and Computational Geometry ap- pears 20 years after the (cid:12)rst edition. A generation of researchers and practitioners have learned their craft using the Handbook. Since the publication of the 2nd edi- tion in 2004, the (cid:12)eld has grown substantially. New methods have been developed tosolvelongstandingopenproblems,withmanysuccesses. Advancesintechnology demanded new computational paradigms in processing geometric data. Fifty-eight out of sixty-(cid:12)ve chapters from the second edition of the Handbook have been revised and updated, many of them by the original authors, others in collaboration with new co-authors. Ten new chapters have been added. Five of the new chapters are devoted to computational topology and its applications. A new chapter on proximity algorithms gives a comprehensive treatment of relative neighbohood graphs and geometric spanners. New chapters on coresets, sketches, "-nets, and "-approximations cover geometric methods to cope with large data. Twonew chaptersexpandon therecentbreakthroughsin rigiditytheory. Wehope the Handbook will remain a one-stop reference book, accessible to both specialists and nonspecialists. Wewouldliketothankthefollowing,onbehalfofthe103authorsandourselves, for providing valuable feedback on the preliminary version of various chapters: BernardoA(cid:19)brego,AbdoAlfakih,ImreB(cid:19)ar(cid:19)any,S(cid:127)orenLennartBerg,M(cid:19)onicaBlanco, Prosenjit Bose, Mireille Bousquet-M(cid:19)elou, Kevin Buchin, Steve Butler, Sergio Ca- bello,PabloCamara,GunnarCarlsson,CesarCeballos,JosephChan,DannyChen, Siu-WingCheng,VincentCohen-Addad,M(cid:19)onikaCsik(cid:19)os,AntoineDeza,KevinEm- mett, Jeff Erickson, Silvia Fern(cid:19)andez-Merchant, Moritz Firsching, Florian Frick, Xavier Goaoc, Steven Gortler, Christian Haase, Bill Jackson, Bruno Jartoux, Iyad Kanj, Matya Katz, Bal(cid:19)azs Keszegh, Hossein Khiabanian, Csaba Kir(cid:19)aly, Matias Korman, Francis Lazarus, Michael Lesnick, Michael Mandell, Arnaud de Mesmay, TobyMitchell,EranNevo,PatrickvanNieuwenhuizen,ArnauPadrol,ValentinPol- ishchuk,HannesPollehn,DanielRosenbloom,RamanSanyal,PaulSeiferth,Michiel Smid, Yannik Stein, Jean Taylor, Louis Theran, Dimitrios Thilikos, Haitao Wang, Andrew Winslow, Richard Wolff, Ge Xia, and Sakellarios Zairis. Jacob E. Goodman Joseph O’Rourke Csaba D. T(cid:19)oth i i i i i i \K25063" | 2017/9/15 | 18:58 | page 1 | i i Part I COMBINATORIAL AND DISCRETE GEOMETRY 1 i i i i i i \K25063" | 2017/9/15 | 18:58 | page 3 | i i 1 FINITE POINT CONFIGURATIONS J(cid:19)anos Pach INTRODUCTION The study of combinatorial properties of (cid:12)nite point con(cid:12)gurations is a vast area of research in geometry, whose origins go back at least to the ancient Greeks. Since it includes virtually all problems starting with \consider a set of n points in space," space limitations impose the necessity of making choices. As a result, we will restrict our attention to Euclidean spaces and will discuss problems that we (cid:12)nd particularly important. The chapter is partitioned into incidence problems (Section 1.1), metric problems (Section 1.2), and coloring problems (Section 1.3). 1.1 INCIDENCE PROBLEMS Inthissectionwewillbeconcernedmainlywiththestructureofincidencesbetween a(cid:12)nitepointcon(cid:12)gurationP andasetof(cid:12)nitelymanylines(or,moregenerally,k- dimensional (cid:13)ats, spheres, etc.). Sometimes this set consists of all lines connecting theelementsofP. TheprototypeofsuchaquestionwasraisedbySylvester[Syl93] more than one hundred years ago: Is it true that for any con(cid:12)guration of (cid:12)nitely manypointsintheplane,notallonaline,thereisalinepassingthroughexactlytwo points? This problem was rediscovered by Erd}os [Erd43], and affirmative answers to this question was were given by Gallai and others [Ste44]. Generalizations for circlesandconicsectionsinplaceoflineswereestablishedbyMotzkin[Mot51]and Wilson-Wiseman [WW88], respectively. GLOSSARY Incidence: A point of con(cid:12)guration P lies on an element of a given collection of lines (k-(cid:13)ats, spheres, etc.). Simple crossing: Apointincidentwithexactlytwoelementsofagivencollection of lines or circles. Ordinary line: A line passing through exactly two elements of a given point con(cid:12)guration. Ordinary circle: Acirclepassingthroughexactlythreeelementsofagivenpoint con(cid:12)guration. Ordinary hyperplane: A (d(cid:0)1)-dimensional (cid:13)at passing through exactly d ele- ments of a point con(cid:12)guration in Euclidean d-space. Motzkin hyperplane: A hyperplane whose intersection with a given d-dimen- sional point con(cid:12)guration lies|with the exception of exactly one point|in a (d(cid:0)2)-dimensional (cid:13)at. 3 i i i i

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