# Extremal Problems for Geometric Hypergraphs

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Extremal Problems for Geometric Hypergraphs
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ExtremalProblemsforGeometricHypergraphs  1  ExtremalProblemsforGeometricHypergraphs  TamalK.Dey  1  andJ anosPach  2  Abstract  A  geometrichypergraph  H  isacollectionof  i  -dimensionalsimplices,called  hyperedges  or,simply, edges  ,inducedbysome(  i  +1)-tuplesofa  vertexset  V  ingeneralpositionin  d  -space.The topologicalstructureofgeometric  graphs  ,i.e.,thecase  d  =2  ;i  =1,hasbeenstudiedextensively,anditprovedtobeinstrumentalforthesolutionofawiderangeofproblemsincombinatorialand computationalgeometry.Theyincludethe  k  -setproblem,proximityquestions,boundingthenum- berofincidencesbetweenpointsandlines,designingvariousecientgraphdrawingalgorithms,etc.Inthispaper,wemakeanattempttogeneralizesomeofthesetoolstohigherdimensions.Wewillmainlyconsiderextremalproblemsofthefollowingtype.Whatisthelargestnumberof edges(  i  -simplices)thatageometrichypergraphof  n  verticescanhavewithoutcontainingcertain  forbidden  congurations?Inparticular,wediscussthespecialcaseswhentheforbiddencongura- tionsare  k  intersectingedges, k  pairwiseintersectingedges, k  crossingedges, k  pairwisecrossing edges, k  edgesthatcanbestabbedbyan  i  -at,etc.Someofourestimateswillbetight. 1Introduction  Inrecentyears,thestudyofgraphdrawingshasbecomearichseparatedisciplinewithincomputational geometry.Muchoftheresearchhasbeenmotivatedbyapplications,includingsoftwareengineering, CAD,databasedesign,cartography,circuitschematics,automaticanimation,visualinterfaces,etc. (See27].)Itisquiteremarkablethatclassicalgraphtheoryprovedtoberatherpowerlesstotackle manyofthearisingproblems.Instead,oneoftenhadtodevelopnewtopologicaltoolstodealwith familiesofcurves,i.e.,graphsdrawnintheplaneorinsomeothersurfaces.Perhapsthebestknown exampleistheLipton{TarjanSeparatorTheoremforplanargraphs15],whichhasmanyextensions, generalizations,andabroadspectrumofapplicationsrangingfromnumericalanalysistocomplexity theory.Inparticular,itenablesustousethedivide-and-conquerparadigmtoconstructvarious geometricrepresentationsofabstractgraphsandnetworks.Anotherimportantexampleisthefollowing resultdiscoveredindependentlybyAjtai-Chv atal-Newborn-SzemerediandLeighton.Itcanbeused toobtaine.g.sharpboundsforthearearequirementofgraphlayouts.Let    (  G  )denotethe  crossing number  ofagraph  G  ,i.e.,theminimumnumberofcrossingpairsofedgesoverallplanardrawingsof  G  .  1  Dept.ofCSE,IIT,Kharagpur,India721302  2  MathematicalInstituteoftheHungarianAcademyofSciencesandCourantInstituteofMathematicalSciences,New YorkUniversity,NewYork,NY10012.ResearchsupportedbyNSFgrantCCR-94-24398,PSC-CUNYResearchAward 663472,andOTKA-4269.  ExtremalProblemsforGeometricHypergraphs  2  Theorem1.1  114Let  G  beasimplegraphwith  n  verticesand  e  (  G  )  edgesIf  e  (  G  )    4  n  then    (  G  )    e  (  G  )  3  100  n  2  AsSzekely24]pointedout,thisresultalmostimmediatelyimpliestheSzemeredi-Trottertheorem 25],26]onthenumberofincidencesbetweenpointsandlines.Hisargumentissoniceandshort thatwecannotresistadaptingittoestablishthefollowinggeneralizationoftheSzemeredi-Trotterthe- orem,whichwasfoundbyClarkson-Edelsbrunner-Guibas-Sharir-Welzlandhasnumerousalgorithmic consequences.(ForsomeotherapplicationsofSzekely'sidea,see19].)  Theorem1.2  8Thetotalnumberofsidesof  n  distinctcellsdeterminedby  m  linesingeneral positionintheplaneisatmost  O  (  m  2  =  3  n  2  =  3  +  m  )  Proof.  Noticethatitissucienttoprovetheassertionforasystemofcells  C  ,notwoofwhichshare anedge.Pickapoint  p  i  ineachcell  c  i  2C  .Foranypair(  s  i  ;s  j  )ofcollinearedgesbelongingto  c  i  2C  and  c  j  2C  ,respectively,connect  p  i  to  p  j  byapolygonalchainoflengththreeviathemidpointsof thesegments  s  i  and  s  j  ,providedthatthispolygonisnotadjacenttoanyothermemberof  C  .The collectionofthesepolygonalchainscanberegardedastheedgesetofagraph  G  0  whosevertices are  p  1  ;:::;p  n  .Ifalineisadjacentto  k  cellsin  C  ,thenitcontributestoexactly  k  ?  1edgesof  G  0  . Hence,  X  ,thetotalnumberofsidesofallcellsin  C  ,diersfromthenumberofedgesof  G  0  byatmost  m  .Removingthemultipleedgesfrom  G  0  ,weobtainasimplegraphwhosenumberofedgessatises  e  (  G  )    e  (  G  0  )  =  4    (  X  ?  m  )  =  4.Inviewofthefactthatanycrossingbetweentwoedgesof  G  occursat acrossingofsomepairoflinesofthearrangement,Theorem1.1impliesthateither  e  (  G  )  <  4  n  or    m  2  !      (  G  )    e  (  G  )  3  100  n  2  :  Since  X    4  e  (  G  )+  m  ,Theorem1.2follows. Givenasetof  n  pointsingeneralpositionintheplane,jointwoofthembyalinesegmentif thereareexactly  k  pointsononesideofthelineconnectingthem.Let  G  denotetheresultinggraph. Lov asz16]provedthatnostraightlinecancrossmorethan2  k  edgesof  G  .NowTheorem1.1implies thatthenumberofedges(thenumberofsocalled  k  sets  )isatmost  O  (  k  1  =  2  n  ).Indeed,ifthenumber ofedgesis  e  ,theneitherwehave  e<  4  n  orthereexistsanedgecrossingatleast  e  2  =  (50  n  2  )other edges.Thus,  e  2  =  (50  n  2  )    2  k  ,asrequired.(See21]foraslightimprovement.)Itwasshownby B ar any-F uredi-Lov asz6]thatasimilarapproachcanbeusedtoestablishanon-trivialupperbound  onthenumberof  halvingplanes  in3-space,whichwassubsequentlyimprovedto  O  (  n  8  =  3  )in4],10]. Agraphdrawnintheplanebypossiblycrossingstraight-linesegmentsiscalleda  geometricgraph  . Moreprecisely,ageometricgraph  G  consistsofasetofpoints  V  ingeneralpositionintheplane andasetofsegments  E  whoseendpointsbelongto  V  .Aswasdemonstratedabove,foranumberof applicationsitwasnecessarytosolvesomeextremalproblemsforgeometricgraphs.Thesystematic studyoftheseproblemswasinitiatedbyP.Erd} os,Y.Kupitz13],andM.Perles.(Forarecentsurvey, see18].) Itseemsplausiblethattoextendtheincidenceresultstohigherdimensions,toimprovetheupper boundforthenumberoftimestheunitdistancecanoccuramong  n  pointsin3-space,ortomake furtherprogressconcerningthe  k  -setproblem,onehastondtherightgeneralizationsofTheorem 1.1tosystemsofsurfacesorsurfacepatchesin  d  -space.Forsimplicity,wewillonlydiscussthecase whenthesesurfacepatchesareat(simplices).   ExtremalProblemsforGeometricHypergraphs  3  Denition1.1  A  d  -dimensionalgeometric  r  -hypergraph  H  d r  isapair  (  V;E  )  where  V  isasetof pointsingeneralpositionin  <  d  and  E  isasetofclosed  (  r  ?  1)  dimensionalsimplicesinducedby some  r  tuplesof  V  Thesets  V  and  E  arecalledthe  vertexset  and  edgeset  of  H  d r  respectively  Someearlyresultsforcompletegeometrichypergraphs  H  d d  +1  wereestablishedin5],7].Akiyama andAlon3]provedthefollowingtheorem.Let  V  =  V  1    :::    V  d  (  j V  1  j =  :::  =  j V  d  j =  n  )bea  dn  -elementsetingeneralpositionin  <  d  ,andlet  E  consistofall(  d  ?  1)-dimensionalsimpliceshaving exactlyonevertexineach  V  i  .Then  E  contains  n  disjointsimplices.Combiningthiswitharesultof Erd} os11],weobtainanon-trivialupperboundforthenumberofedgesofa  d  -dimensionalgeometric  d  -hypergraphof  n  verticesthatcontainsno  k  pairwisedisjointedges. Ifwewanttoexclude  crossings  ratherthandisjointedges,orwanttogeneralizeTheorem1.1to geometrichypergraphs,wefacethefollowingproblem.Evenifwerestrictourattentiontosystems oftrianglesinducedby3-dimensionalpointsetsingeneralposition,itisnotcompletelyclearhowa \crossing"shouldbedened,letalonethenotionof\crossingnumber".Iftwosegmentscross,they donotshareanendpoint.Shouldthisremaintruefortriangles?Wehavetoclarifytheterminology.  Denition1.2  Twosimplicesaresaidtohavea  non-trivialintersection  iftheirrelativeinteriors haveapointincommonIfinadditionthetwosimplicesarevertexdisjointthentheyaresaidto  cross  Moregenerally  k  simplicesaresaidtohavea  non-trivialintersection  iftheirrelativeinteriors haveapointincommonIfinadditionallsimplicesarevertexdisjointthentheyaresaidto  cross Consider  k  simplices.Itisimportanttonotethatthefactthat  everypair  ofthemhasanon-trivial intersectiondoesnotimplythat  all  ofthemdo.Toemphasizethatthisstrongerconditionissatised, weoftensaythatthesimpliceshavea  nontrivialintersectioninthestrongsense  ,orsimplythatthey  stronglyintersect  .Similarly,asetof  pairwise  crossingsimplicesisnotnecessarily  crossing  .Ifwant toemphasizethatthey  all  cross,wewillsaythatthey  crossinthestrongsense  ,orshortlythatthey  stronglycross  . Aswepickmoreandmoredistinct(  r  ?  1)-dimensionalsimplicesinducedbyasetof  n  points in  <  d  ,thenumberofcrossingsbetweenthemwillusuallyincrease.Theaimofthispaperisto generalizetheplanarresultstoobtainsomeinformationaboutthegrowthrateofthisprocess.Inthe inverseformulation,onecanaskforthemaximumnumberofedgesthata  d  -dimensionalgeometric  r  -hypergraph  H  d r  of  n  verticescanhavewithoutcontainingsomexedcrossingpattern.Throughout thispaper,let  f  d r  (  F  ;n  )denotethismaximum,where  F  isthefamilyof   forbidden  congurations,i.e., forbiddengeometricsubhypergraphs.Mostofourboundswillbeasymptotic:  d  and  r  arethoughtto bexed,while  n  tendstoinnity. Inthenexttwosections,weestimate  f  d r  (  F  ;n  )forvariousfamilies  F  .InSection4,wegeneralize Theorem1.1.Finally,wediscusssomerelatedquestionsandgiveafewapplicationsofourresults.  2Fulldimensionalsimplices  Let  I  r k  (resp.  SI  r k  )denotetheclassofallgeometrichypergraphsconsistingof  k  (  r  ?  1)-dimensional simplices,anytwoofwhichhaveanon-trivialintersection(resp.allofwhicharestronglyintersecting). Similarly,let  C  r k  (resp.  SC  r k  )denotetheclassofallgeometrichypergraphsconsistingof  k  pairwise crossing(resp.stronglycrossing)(  r  ?  1)-simplices.   ExtremalProblemsforGeometricHypergraphs  4  Theorem2.1  Foranyxed  k>  1  onecanselectatmost  O  (  n  d  d=  2  e  )  d  dimensionalsimplicesinduced by  n  pointsin  d  spacewiththepropertythatno  k  ofthemshareacommoninteriorpointThisbound cannotbeimprovedThatis  f  d d  +1  (  I  d  +1  k  ;n  )=(  n  d  d=  2  e  )  ;f  d d  +1  (  SI  d  +1  k  ;n  )=(  n  d  d=  2  e  )  :  Proof.  Clearly,wehave (  n  d  d=  2  e  )    f  d d  +1  (  I  d  +1  k  ;n  )    f  d d  +1  (  SI  d  +1  k  ;n  )  ;  wheretherstinequalityfollowsfromthefactthattherearetriangulationsofsize(  n  d  d=  2  e  )with  n  verticesin  <  d  .Considere.g.theverticalprojectionofthelowerpartofanycyclicpolytopeof  n  verticesin  <  d  +1  . Toseethat  f  d d  +1  (  SI  d  +1  k  ;n  )    O  (  n  d  d=  2  e  ),wesetupachargingscheme.Letusregard  <  d  ?  1  asthe coordinatehyperplanein  <  d  spannedbytherst  d  ?  1axes,andlet  X  d  denotethelastcoordinate axis.Supposethat  X  d  isvertical.Fixageometrichypergraph  H  d d  +1  =(  V;E  )whichhasno  k  edges withacommoninteriorpointandwhose  n  verticesareingeneralposition.Forany  `  -dimensional simplexinducedby  V  ,where  `  b  (  d  ?  1)  =  2  c  ,let  E      E  denotethesetofalledgesof  H  d d  +1  that containontheirboundaries.Itfollowsfromtheconditionon  H  d d  +1  thattheinniteverticalcylinder +  X  d  basedonintersectsatmost2(  k  ?  1)elementsof  E    .Letuschargeoneunitforeach oftheseedges.Sincethetotalnumberof  `  -simpliceswith  `  b  (  d  ?  1)  =  2  c  isatmost  d  d=  2  e  n  d  d=  2  e  , itremainstoshowthateveryedge  e  2  E  hasbeenchargedfor.Indeed,byRadon'stheorem23], thevertexsetoftheorthogonalprojectionof  e  into  <  d  ?  1  canbepartitionedintotwoparts,  S  1  and  S  2  ,suchthattheirconvexhullscrosseachotherand  j S  1  j +  j S  2  j =  d  +1.Supposewithoutlossof generalitythat  j S  1  jb  (  d  +1)  =  2  c  .Thentheconvexhullof  S  1  isan  `  -dimensionalsimplex  1  forsome  `  b  (  d  ?  1)  =  2  c  ,andwehadtocharge  1  for  e  .  Theorem2.2  Let  E  beanysetof  d  dimensionalsimplicesinducedbyan  n  elementpointset  V  <  d  If  E  hasnotwocrossingelementsthen  j E  j =  O  (  n  d  )  andthisboundisasymptoticallytightIn notation  f  d d  +1  (  C  d  +1 2  ;n  )=(  n  d  )  :  Proof.  Toprovethelowerbound,considerageometrichypergraphconsistingofall  d  -dimensional simplicesinducedby  V  thatcontainagivenvertex  v  2  V  . Nextweestablishtheupperbound.If  E  hasnotwosimpliceshavinganon-trivialintersection, then  j E  j  O  (  n  d  d=  2  e  ),bytheprevioustheorem.Otherwise,choosetwo  d  -simplices  1  ;    2  2  E  whose intersectionisnon-trivial.Itiseasytoshow(seee.g.9],10])thatthereexistan  `  1  -face  0  1  of  1  andan  `  2  -face  0  2  of  2  with  `  1  +  `  2  =  d  suchthat  0  1  and  0  2  arecrossing. Assumerstthatthereisanedge  e  2  E  whichisvertexdisjointfrom  0  1  andcontains  0  2  asa face.Theneveryedge  f  2  E  thatcontains  0  1  asafacemustshareatleastonevertexwith  e  .The numberofsuchsimplices  f  isatmost(  d  +1)  ?  n d  ?  `  1  ?  1    .Letusremoveallofthemfrom  E  . Inthesecondcase,everyedge  e  2  E  thatcontains  0  2  asafacesharesavertexwith  0  1  .Obviously, thenumberofsuchsimplices  e  isatmost(  `  1  +1)  ?  n d  ?  `  2  ?  1    .Removeallofthemfrom  E  .   ExtremalProblemsforGeometricHypergraphs  5 Wecontinuethisprocedureuntilthereremainnonon-trivialintersectionsin  E  .Atthispoint,  E  hasatmost  O  (  n  d  d=  2  e  )elements,andthetotalnumberofsimplicesthathavebeenremovedisatmost    n `  1  +1  !  (  d  +1)    n d  ?  `  1  ?  1  !  +    n `  2  +1  !  (  `  1  +1)    n d  ?  `  2  ?  1  !  =  O  (  n  d  )  :  3  (  d  ?  1)  -simplicesin  d  -space  Theorem3.1  Let  E  beafamilyof  (  d  ?  1)  dimensionalsimplicesinducedbyan  n  elementpointset  V  <  d  suchthat  E  hasno  k  memberswithpairwisenontrivialintersections  d;k>  1  Thenfor  k  =2  and  3  wehave  j E  j =  O  (  n  d  ?  1  )  Otherwise  j E  j =  O  (  n  d  ?  1  log  2  k  ?  6  n  )  Innotation  f  d d  (  I  d k  ;n  )=  (  O  (  n  d  ?  1  )  if  k  =2  ;  3  O  (  n  d  ?  1  log  2  k  ?  6  n  )  otherwise Thisresultisasymptoticallytightif  d;k    3  Proof.  For  d  =2,theassertionistrue,bytheresultsof20]and2].Assumethat  d    3.For any(  d  ?  3)-simplexinducedby  V  ,let  E    denotethefamilyofallmembersof  E  thatcontain asaface.Pickanypoint  p    intherelativeinteriorof,andlet  F    denotethe3-dimensionalat orthogonaltoandpassingthrough  p    . Every  e  2  E    meets  F    inatriangle,whosetwosidesincidentto  p    aretheintersectionsof  F    withthetwo(  d  ?  2)-facesof  e  containing.Thus,thetotalnumberofsidesincidentto  p    that occurinsome  e  \  F    (  e  2  E    )isatmost  n  ?  d  +2  <n  .Takeasmall2-dimensionalsphere  S  2    F    centeredat  p    .Theintersectionsof  S  2  withtheelementsof  E    formtheedgesetofagraphwithat most  n  vertices.Itfollowsfromthepropertiesof  E  thatthisgraphhasno  k  pairwisecrossingedges, so,bytheplanarresults,itsnumberofedges,  j E    j ,satises  j E    j =  (  O  (  n  )if  k  =2  ;  3;  O  (  n  log  2  k  ?  6  n  )otherwise. Summingoverall(  d  ?  3)-simplicesinducedby  V  ,weobtain  ?  d  2    j E  j =  P    j E    j ,andhencetheupper bound. Toshowthattheresultistightfor  d  =3  ;k  =2,consideranestedsequenceof  n=  2pyramidsbased onthesame2-dimensionalconvex  n=  2-gon.Thesepyramidshaveatotalof  n  2  =  4triangularfaces,no twoofwhichhaveanon-trivialintersection. Itisanoutstandingopenproblemtodecidewhethertheorderofmagnitudeoftheabovebound canbeimprovede.g.for  d  =4  ;k  =2.However,modifyingtheproceduredescribedintheproof Theorem2.1,onecanshowthatthefollowingrelatedresultisasymptoticallytight.Thedetailsare lefttothereader.
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