Connections in discrete mathematics a celebration of the work of ron graham 1st edition steve butler

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ConnectionsinDiscreteMathematics

Discretemathematicshasbeenrisinginprominenceinthepastfiftyyears,bothasa toolwithpracticalapplicationsandasasourceofnewandinterestingmathematics. Thetopicsindiscretemathematicshavebecomesowelldevelopedthatitiseasyto forgetthatcommonthreadsconnectthedifferentareas,anditisthroughdiscovering andusingtheseconnectionsthatprogressisoftenmade.

Formorethanfiftyyears,RonGrahamhasbeenabletoilluminatesomeofthese connectionsandhashelpedbringthefieldofdiscretemathematicstowhereitistoday. Tocelebratehiscontribution,thisvolumebringstogethermanyofthebestresearchers workingindiscretemathematics,includingFanChung,ErikDemaine,PersiDiaconis, PeterFrankl,AlHales,JeffreyLagarias,AllenKnutson,JanosPach,CarlPomerance, NeilSloane,andofcourseRonGrahamhimself.

stevebutler istheBarbaraJJansonProfessorofMathematicsatIowaState University.Hisresearchinterestsincludespectralgraphtheory,enumerative combinatorics,mathematicsofjuggling,anddiscretegeometry.Heisthecoeditor of TheMathematicsofPaulErd ˝ os

joshuacooper isProfessorofMathematicsattheUniversityofSouthCarolina. Hecurrentlyservesontheeditorialboardof Involve.Hisresearchinterestsinclude spectralhypergraphtheory,linearandmultilinearalgebra,probabilisticcombinatorics, quasirandomness,combinatorialnumbertheory,andcomputationalcomplexity.

glennhurlbert isProfessorandChairoftheDepartmentofMathematicsand AppliedMathematicsatVirginiaCommonwealthUniversity.Hisresearchinterests includeuniversalcycles,extremalsettheory,combinatorialoptimization, combinatorialbijections,andmathematicaleducation,andheisrecognizedasa leaderinthefieldofgraphpebbling.

ConnectionsinDiscreteMathematics

ACelebrationoftheWorkofRonGraham

IowaStateUniversity

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ListofContributors

1ProbabilizingFibonacciNumbers

PersiDiaconis

2OntheNumberofONCellsinCellularAutomata

N.J.A.Sloane

3SearchforUltraflatPolynomialswithPlusandMinusOne Coefficients

AndrewOdlyzko 4GeneralizedGon ˇ carovPolynomials

RudolphLorentz,SalvatoreTringali,andCatherineH.Yan

5TheDigraphDropPolynomial

FanChungandRonGraham

6UnramifiedGraphCoversofFiniteDegree

Hau-WenHuangandWen-ChingWinnieLi

7TheFirstFunctionandItsIterates

CarlPomerance

8Erd ˝ os,Klarner,andthe 3x + 1 Problem

JeffreyC.Lagarias

9AShortProofforanExtensionoftheErd ˝ os–Ko–Rado Theorem

PeterFranklandAndreyKupavskii

10TheHaight–RuzsaMethodforSetswithMoreDifferences thanMultipleSums 173

MelvynB.Nathanson

11DimensionandCutVertices:AnApplicationofRamsey Theory 187

WilliamT.Trotter,BartoszWalczak,andRuidongWang

12RecentResultsonPartitionRegularityofInfiniteMatrices

NeilHindman

13SomeRemarkson π

ChristianReiher,Vojt ˇ echRödl,andMathiasSchacht

14RamseyClasseswithClosureOperations(Selected CombinatorialApplications)

JanHubi ˇ ckaandJaroslavNešetˇril

15BorsukandRamseyTypeQuestionsinEuclideanSpace 259

PeterFrankl,JánosPach,ChristianReiher,andVojt ˇ echRödl

16Pick’sTheoremandSumsofLatticePoints

KarlLevyandMelvynB.Nathanson

17ApollonianRingPackings

AdrianBolt,SteveButler,andEspenHovland

18JugglingandCardShufflingMeetMathematicalFonts

ErikD.DemaineandMartinL.Demaine

19RandomlyJugglingBackwards

AllenKnutson

20ExplicitErrorBoundsforLatticeEdgeworthExpansions 321

JoeP.Buhler,AnthonyC.Gamst,RonGraham,and AlfredW.Hales

Contributors

AdrianBolt

IowaStateUniversity,Ames,IA50011,USA

JoeP.Buhler

CenterforCommunicationsResearch,SanDiego,CA92121,USA

SteveButler

IowaStateUniversity,Ames,IA50011,USA

FanChung

UniversityofCaliforniaatSanDiego,LaJolla,CA92093,USA

ErikD.Demaine

MITComputerScienceandArtificialIntelligenceLaboratory,Cambridge, MA02139,USA

MartinL.Demaine

MITComputerScienceandArtificialIntelligenceLaboratory,Cambridge, MA02139,USA

PersiDiaconis

DepartmentsofMathematicsandStatistics,StanfordUniversity,Stanford,CA 94305,USA

PeterFrankl

AlfrédRényiInstituteofMathematics,HungarianAcademyofSciences, H-1053Budapest,Hungary

Contributors

AnthonyC.Gamst

CenterforCommunicationsResearch,SanDiego,CA92121,USA

RonGraham UniversityofCaliforniaatSanDiego,LaJolla,CA92093,USA

AlfredW.Hales

CenterforCommunicationsResearch,SanDiego,CA92121,USA

NeilHindman

DepartmentofMathematics,HowardUniversity,Washington,DC20059, USA

EspenHovland

IowaStateUniversity,Ames,IA50011,USA

Hau-WenHuang

DepartmentofMathematics,NationalCentralUniversity,Chung-Li32001, Taiwan

JanHubi ˇ cka

ComputerScienceInstituteofCharlesUniversity(IUUK),CharlesUniversity, 11800Praha,CzechRepublic

AllenKnutson CornellUniversity,Ithaca,NY14853,USA

AndreyKupavskii

MoscowInstituteofPhysicsandTechnology,Dolgobrudny,MoscowRegion, 141701,RussianFederation;andUniversityofBirmingham,Birmingham, B152TT,UK

JeffreyC.Lagarias

DepartmentofMathematics,UniversityofMichigan,AnnArbor,MI 48109–1043,USA

KarlLevy

DepartmentofMathematics,BoroughofManhattanCommunityCollege (CUNY),NewYork,NY10007,USA

Wen-ChingWinnieLi

DepartmentofMathematics,PennsylvaniaStateUniversity,UniversityPark, PA16802,USA

RudolphLorentz

ScienceProgram,TexasA&MUniversityatQatar,Doha,Qatar

MelvynB.Nathanson

DepartmentofMathematics,LehmanCollege(CUNY),Bronx,NY10468, USA

JaroslavNešet ˇ ril

ComputerScienceInstituteofCharlesUniversity(IUUK),CharlesUniversity, 11800Praha,CzechRepublic

AndrewOdlyzko

SchoolofMathematics,UniversityofMinnesota,Minneapolis,MN55455, USA

JánosPach

RényiInstituteandEPFL,Station8,CH-1014Lausanne,Switzerland

CarlPomerance

MathematicsDepartment,DartmouthCollege,Hanover,NH03755,USA

ChristianReiher

FachbereichMathematik,UniversitätHamburg,20146Hamburg,Germany

Vojt ˇ echRödl

DepartmentofMathematicsandComputerScience,EmoryUniversity, Atlanta,GA30322,USA

MathiasSchacht

FachbereichMathematik,UniversitätHamburg,20146Hamburg,Germany

N.J.A.Sloane

TheOEISFoundationInc.,11SouthAdelaideAve.,HighlandPark,NJ 08904,USA

x Contributors

SalvatoreTringali

InstituteforMathematicsandScientificComputing,UniversityofGraz, NAWIGraz,Heinrichstr.36,8010Graz,Austria

WilliamT.Trotter

SchoolofMathematics,GeorgiaInstituteofTechnology,Atlanta,GA30332, USA

BartoszWalczak

TheoreticalComputerScienceDepartment,FacultyofMathematicsand ComputerScience,JagiellonianUniversity,Kraków30-348,Poland

RuidongWang BlizzardEntertainment,Irvine,CA92618,USA

CatherineH.Yan

DepartmentofMathematics,TexasA&MUniversity,CollegeStationTX 77845,USA

Preface

Thelastfiftyyearshaveseenrapidgrowthintheriseofdiscretemathematics, inareasrangingfromtheclassicsofnumbertheoryandgeometrytothemoderntoolsincomputationandalgorithms,withhundredsoftopicsinbetween. Partofthisgrowthisdrivenbytheincreasingavailabilityandimportanceof computationalpower,andpartisduetotheguidinginfluenceandleadership ofmathematiciansinthisfieldwhohavehelpedtoencouragegenerationsof mathematicianstopursueresearchinthisarea.

Amongthesemathematicianswhohaveplayedaleadershiprole,RonGrahamstandsoutforhiscontributionstotheory,hisvisibilitytothelargercommunity,hisroleinmentoringmanyyoungmathematicians,andforhislongevity.

In1962,RonGrahamfinishedhisdissertationincombinatorialnumbertheoryundertheleadershipofDerrickLehmer.HesoonfoundhimselfatBell Labs,wherehewouldspendthenextthirty-sevenyears,includingasdirectoroftheMathematicalSciencesResearchCenterandasChiefScientistof AT&TLabs–Research,beforehis(first)retirementin1999.Duringthistime hehelpedbringtogethermanyofthebestandbrightestyoungmindsindiscrete mathematicstoworkintheLabsandwouldguidetheirresearchandprepare themfortheirlatercareers.AfterBellLabs,RonwenttoUniversityofCalifornia,SanDiegotobecometheIrwinandJoanJacobsChairinInformationand ComputerScienceintheDepartmentofComputerScienceandEngineering untilhis(second)retirementin2016.HeisnowemeritusprofessoratUniversityofCalifornia,SanDiegoandremainsmathematicallyactive.

Overthepast50years,hehashadaconstantflowofnewideas,withmore than300papers,ahalfdozenbooks,andhundredsofeditorialassignments.He hasalsotraveledtheworldgivingtalksonmathematics(sometimesdemonstratingsomeofhisacrobaticskillssuchasone-armedhandstandsandjuggling,showingstudentsthatmathematiciansaren’tthestereotypicalintroverts). ThankstohisfriendshipwithPaulErd ˝ os(30jointpapers!)andhisfrequent

travel,RonplayedanimportantroleasabridgetomathematicsinHungary andseveralothercountriesatatimewhencommunicationandcollaboration werelimited.AspresidentoftheAmericanMathematicalSocietyandlaterthe MathematicalAssociationofAmerica,Ronledthelargestmathematicalsocietiesintheworld.Inadditiontoallofthis,RonGrahamhasbeenrecognized withnumerousawards,accolades,andhonorarydegrees.Itishardtobelieve thatoneindividualwasabletoaccomplishsomuch,butasRonlikestopoint out“therearetwenty-fourhoursintheday,andifthat’snotenough,thereare alsothenights.”

In2015,RonGrahamturnedeighty,andtohelpmarkthisoccasionaspecial conferencewasorganized, ConnectionsinDiscreteMathematics.Thiswasa chancetobringtogethermanyofhisfriendsandcolleagues,thebestandbrightestindiscretemathematics,tocelebrateRon,andalsotocelebratediscrete mathematics.Amajorthemeoftheconferencewasconnections,boththepersonalconnections(asRonhadwithsomanyspeakersandparticipants)aswell astheconnectionsbetweenmathematicaltopics.Bothtypesofconnectionsare whatleadtoadvancesinmathematicsandopenupnewideasforexploration.

Thisbookcameoutoftheconference,withmanyoftheauthorshaving beenfeaturedspeakers.Thechaptershereareacrossthespectrumofdiscrete mathematics,withtopicsinnumbertheory,probability,graphtheory,Ramsey theory,discretegeometry,algebraiccombinatorics,and,ofcourse,juggling.A beautifulmixoftopicsandalsoofwritingstyles,thisbookhassomethingfor everyone.

Wethankthemanyauthorsfortheirexcellentcontributions,includingJoe Buhler,FanChung,ErikDemaine,PersiDiaconis,PeterFrankl,AlHales, JeffreyLagarias,AllenKnutson,JarikNešetˇril,JanosPach,CarlPomerance, Vojt ˇ echRödl,NeilSloane,TomTrotter,CatherineYan,andofcourseRonGraham.Wewerehappytoseethewealthofideasthatwereinthesechaptersand hopethatreaderswillfindsomethingthatinspiresthem.

AllthreeoftheeditorshavebeenheavilyinfluencedbyRonGrahamand hisfriendship,andlikemanypeopleinourfield,wewouldnotbewherewe aretodaywithouthim.Thankyou,Ron.

Fig.0.1.RonGrahamdemonstratingalargeRubik’scubetoC.K.Cheng,Kevin Milans,andJoelSpenceratthe ConnectionsinDiscreteMathematics conference inJune2015.ImagecourtesyofIRMACS.Usedwithpermission.

1

ProbabilizingFibonacciNumbers

PersiDiaconis

Abstract

Thequestion,“WhatdoesatypicalFibonaccinumberlooklike?”leadstointeresting(andimpossible)mathematicsandmanyataleaboutRonGraham.

IhavetalkedtoRonGrahameveryday,moreorless,forthepast43years. Somedayswemissacallbutsomedaysit’sthreeorfourcalls;so“onceaday” seemsaboutright.Therearemanyreasons:hetellsbadjokes,solvesmymath problems,teachesmethings,andismypal.Thefollowingtriestocapturea fewminuteswithRon.

1.1AFibonacciMorning

ThistermI’mteachingundergraduatenumbertheoryatStanford’sDepartment ofMathematics.It’sacourseforbeginningmathmajorsandthestartispretty dry(I’minWeek1):everyintegerisdivisiblebyaprime;if p divides ab then either p divides a or p divides b;uniquefactorization.I’musingBillLeVeque’s fine FundamentalsofNumberTheory.It’sclear,correct,andcheap(aDover paperback).HementionstheFibonaccinumbersandIdecidetospendsome timetheretoliventhingsup.

Fibonaccinumbershavea“crankmath”aspectbuttheyarealsoserious stuff–fromsunflowerseedsthroughHilbert’stenthproblem.Mywayofunderstandinganythingistoask,“‘Whatdoesatypicalonelooklike?”Okay,what doesatypicalFibonaccinumberlooklike?Howmanyareeven?Whataboutthe decompositionintoprimepowers?ArethereinfinitelymanyprimeFibonacci numbers?IrealizethatIdon’tknowand,itturnsout,formostofthesequestions,nobodyknows.ThisistwohoursbeforeclassandIdowhatIalwaysdo: callRon.

“Hey,doweknowwhatproportionofFibonaccinumbersareeven?” “Sure,”hesayswithoutmissingabeat.“It’s 1/3,andit’seasy:letmeexplain. Ifyouwritethemout,

youseethateverythirdoneiseven,andit’seasytoseefromtherecurrence.” Similarly,everyfourthoneisamultipleofthree,so 1/4 aredivisibleby3.Inote thattheperiodforfiveis5,ruiningmyguessatthepattern.Hetellsmethat afterfive,theperiodfortheprime p isadivisorof p ± 1;so 1/8 ofallthe Fn are divisiblebyseven.Actually,thingsareabitmoresubtle.Foranyprime p,the sequence ofFibonaccinumbers(mod p)isperiodic.Let’sstartthemat F0 = 0, when p = 3:

Thelengthoftheperiodiscalledthe Pisanoperiod (withitsownWikipedia page).Theperiodofthreeis8andtherearetwozeros,so 1/4 aredivisibleby 3; 3/8 are1(mod3)and 3/8 are2(mod3).Theseperiodsturnouttobepretty chaoticandmuchisconjectural.Therestofmyquestions,e.g.,is Fn prime infinitelyoften,areworse:“notinourlifetimes,”(Ronsays)Erd ˝ ossaid.

GoingbacktomyphoneconversationwithRon,hesays: Here’ssomethingyourkidscando:YouknowtheFibonaccinumbersgrowpretty fast.Thismeansthat ∞ n=0 1/F2n convergestoitslimitingvalueveryfast.Itturns outtobeaquadraticirrational(!) and youcanshowthatifanumberhasamore rapidlyconvergingrationalapproximation,it’stranscendental.(!)

Andthenhesays,“Here’saneasieroneforyourkids:askthemtoaddup Fn /10n ,1 ≤ n < ∞.”Answer: 10/89 (!)

ItturnsoutthatRonhadworkedonmyquestionsbefore.A1964paper[7] starts,“Let S (L0 , L1 ) = L0 , L1 , L2 ,... bethesequenceofintegerswhichsatisfytherecurrence Ln+2 = Ln+1 + Ln , n = 0, 1, 2,... .Itisclearthatthevalues L0 and L1 determine S (L0 , L1 ),e.g., L (0, 1)isjustthesequenceofFibonacci numbers.Itisnotknownwhetherornotinfinitelymanyprimesoccurin S (0, 1) ….”Hegoesontofindanopposite:apair L0 , L1 sothat no primesoccurin S (L0 , L1 ).Hisbestsolutionwas

L0 = 331635635998274737472200656430763

L1 = 1510028911088401971189590305498785

ThiswassubsequentlyimprovedbyKnuthandthenWilf.Theproblemitself nowhasitsownWikipediapage;searchfor“primefreesequence.”Thecurrent

recordis

L0 = 106276436867

L1 = 35256392432

duetoVesemirsov.

Findingsuchsequencesisrelatedtoproblemssuchas,“Iseveryodd numberthesumofaprimeplusapowerof2?”Theanswerisno;indeed Erdös[6]foundarithmeticprogressionswithnonumbersofthisform.For this,hecreatedthetopic/toolof“coveringcongruences”:asequence {a1 (mod n1 ),..., ak (mod nk )} offinitelymanyresidueclasses {ai + ni x, x ∈ Z} whoseunioncovers Z.Forexample, {0(mod2), 0(mod3), 1(mod4), 5 (mod6), 7(mod12)} isacoveringsetwhereallmoduliaredistinct.Erd ˝ os askedifthereweresuchdistinctcoveringsystemswherethesmallestmodulus–twointheexample–isarbitrarilylarge.Avarietyofnumbertheoryhackers foundexamples.Forinstance,Nicesendfoundasetofmorethan1050 distinct congruenceswithminimummodulus40.OneofRon’sfavoritenegativeresults isatheoremofHough[9]:thereisanabsoluteupperboundtotheminimum modulusofasystemofdistinctcoveringcongruences.TheWikipediaphrase is“coveringsequences.”

Theprecedingparagraphsareamplifiedfromsentencesofthissamephone conversation.RonhasworkedonmathproblemswhereFibonaccifactsforma crucialpartoftheargument“fromthentonow.”Forexample,injointworkwith FanChung[2]theysolvedanoldconjectureofD.J.Newman:forasequence ofnumbers(mod1), x = (x0 , x1 , x2 ,... ),definethestrongdiscrepancy

Theyfoundthefollowing:

Theorem1.1.

ThereaderwholookswillfindFibonaccinumbersthroughouttheproofs; ∞ n=0 1/F2n makesanappearance.

Asapartingshotinourconversation,Ronmovedbacktotheperiodsof FibonacciandLucussequences SL (L0 , L1 )intheprecedingtext.“Youknow, wehadaprettygoodtrickinourbook[4]usingFibonacciperiods.Youshould performitforyourkids.”Letmeperformitforyou.Tounderstandtheconnection,seeDiaconisandGraham[4,p.187].

Theperformerdrawsa4 × 4squareonasheetofpaper.Apredictionis writtenontheback(toownup,it’s49).

Thepattergoesasfollows:“Theyteachkidsthecraziestthingsinschool nowadays.Theotherdaymydaughtercamehometalkingabout‘addingmod seven.’Thatmeansyouaddandtakeawayanythingover7,so5 + 5 = 10 = 3 (mod7).Here,let’stryitout.”Pickanytwosmallnumbers;say5and6are chosen.Writethemdowninpositions(1, 1)and(1, 2).Thensummod7in position(1, 3):

Continueasshown,addingsuccessivepairsrowbyrowuntilallsquaresare filled.Youcanaskspectatorstohelpalongtheway.Attheend,havesomeone (carefully)addupall16numbersintheusualway.Thesumwillmatchyour prediction,49.Ourwrite-upgivespointerstothemathematicalliteratureon Fibonacciperiods.

Inafollow-upcall,ImentionedacharmingfactpointedoutbySusan Holmes.Ifyouwanttoconvertfrommilestokilometers(andback)takethe nextFibonaccinumber(ortheonebefore,togoback).Thus5milesisclose to8kilometers,13milesiscloseto21kilometers,144kilometersiscloseto 89miles,andsoon.Todogeneralnumbers,useZeckendorf’stheorem:any positiveintegercanberepresentedasasumofdistinctFibonaccinumbers: uniquely,ifyouneverusetwoconsecutive Fn .So100 = 89 + 8 + 3,and100 milesisabout144 + 13 + 5 = 162kilometers.(Really,100miles = 160 934 kilometers;it’sonlyanapproximation.)TheZeckendorfrepresentationiseasy tofind,justsubtractoffthelargestpossible Fn eachtime.Formuchmore Fibonacciana,see[8].

1.2ASecondTry

Hereisamoresuccessfulapproachtothequestionofwhatatypical Fn looks like.TakeoneofthemanycodingsofFibonaccinumbersandanswerthe

questionthere.Forexample, Fn countsthenumberofbinarystringsoflength n 2withnotwoconsecutiveones:

F5 = 5 ←→{000, 001, 010, 100, 101}

Let Fn betheFibonaccistringsoflength n.So |Fn |= Fn+2 .Thismismatch innotationisunfortunate,butkeepingtheclassicalnotationfor Fn makesthe literatureeasiertouse.

Forthiscoding,itisnaturaltoask,“Whatdoesatypicalelementof Fn look like?”Throughout,usetheuniformdistribution

Pn (x ) = 1/Fn+2

on Fn .Thisdistributioniswellknowninstatisticalphysicsasthe“hardcore modelin1-D.”Let Xi (x )bethe ithbitof x.Naturalquestionsare:

Whatisthechancethat Xi = 1?

Whatisthedistributionof X1 +···+ Xn ?

Howlongisthelongestzerorunof X1 , X2 ,..., Xn ?

Whatisthewaitingtimedistributionforthefirstone?

Indeed,anyquestionaskedforcointossingcanbeaskedfor Fn .In[5],asimple,efficientalgorithmisgivenforexactgenerationofauniformlychosenelementof Fn (usingtheFibonaccinumbersystem).Ofcourse,thereisliterature onthemixingtimeofthenaturalMarkovchainforgeneratingfromtheuniformdistributionon Fn –pickacoordinateatrandom;trytochangetoits opposite–see[10].

Themainresultsdeveloped(Propositions1.1to1.6):asaprocess, X1 , X2 ,..., Xn isclosetoabinaryMarkovchain ˜ X1 , ˜ X2 ,..., ˜ Xn ,where,with

Theclosenessisintotalvariationfor X1 ,..., Xk with k = n f (n ), f (n ) →∞.Thisisstrongenoughtogiveusefulanswerstothepreviousfour questionsandmanyothers.Letusturnnowtomathematics.

Proposition1.1. Foranyi, 1 ≤ i ≤ n,

Proof. Sequenceswith Xi = 0maybeginwithanyFibonaccisequenceof length i 1(Fi+1 choices)andendwithanyFibonaccisequenceoflength n i (Fn i+2 choices).DividingbythetotalnumberofFibonaccisequences oflength n (Fn+2 )givesthefirstresult.Thesecondissimilar;aoneinposition i forceszerosat i 1, i + 1.Afterthis,thestartandendarearbitraryFibonacci sequences.

Forsubsequentuse,recallthat,with

and Fn istheclosestintegerto φ n /√5.Recall θ = (√5 1)/2.Combining(1.2)andProposition1.1,standardasymptoticsgives

Proposition1.2.

(a)Pn (Xi = 0) = Pn (Xn i+1 = 0), 1 ≤ i ≤ n (symmetry)

(b)Pn (X1 = 0) = θ 1 + O ( φ 2n )

(c)P (Xi = 0) = θ 2

Remark. Part(c)shows,if i and n i arelarge, P (Xi = 0) ∼

Thisofcourseisthestationarydistributionofthetransitionmatrixin(1.2).It isusefultocollecttogetherpropertiesoftheMarkovchain.

Proposition1.3. For θ = (√5 1)/2,letaMarkovchain { ˜ Xn } on {0, 1} have transitionmatrix P = θθ 2 10 , startingdistributionP ( ˜ X1 = 0) = θ ,P ( ˜ X1 = 1) = θ 2 .ThenPhasstationary distribution

=

andPisreversible.Theeigenvaluesare β0 = 1, β1 = θ 1.Therighteigenvectorsare f 0 = 1 1 , f 1 = 1 1/θ 2 .

Ifthechainisdenoted ˜ Xi , 1 ≤ i < ∞,forallnande1 ,..., en ∈{0, 1} anallowablesequence,

Themainresultofthissectiongivesanexplicitboundbetweentheprobabilitydistribution μn,k of X1 ,..., Xk fromtheFibonaccichainand˜μk theprobabilitydistributionoftheMarkovchain ˜ X1 ,..., ˜ Xk asinProposition1.3.The totalvariationdistanceis

Proposition1.4. Withnotationasearlier,forallnand 1 ≤ k ≤ n,

n,k ˜ μk = O ( θ 2(n k ) )

Proof. Asusual,

Forany x1 ,..., xk ,

Itfollowsthat

Theclaimedresultnowfollowsfrom(1.2)inastraightforwardmanner.

Someoftheprecedingquestionshavebeenpreviouslyanswered.Let Sn = X1 +···+ Xn .Diaconis,Graham,andHolmes[5]prove

Proposition1.5.

(a)Pn (Sn = k ) = n+1 k k Fn+2 , 0 ≤ k ≤ n + 1 2 .

(b)En (Sn ) = (n + 1) √5 1 2√5 + 1 5φ + O (nφ 2n ), Var(

/2 dtasntendstoinfinity.

Thelongestzeroruncanbedeterminedbysolvingtheproblemforthe Markovchainandthentransferringitto X1 , X2 ,..., Xn usingProposition1.4.

Proposition1.6. LetMn bethelongestzerorunforauniformelementof Fn ThenMn / log1/θ n → 1 inprobability.

Proof. Proposition1.6followsbyfirstprovingtheparallelresultforthe Markovchain ˜ Xi andthentranferringto Xi usingProposition1.4.Let l = l (n ) = log1/θ n .FromProposition1.3,thetransitionmatrix P isexplicitlydiagonalizedas P = VDV 1 ,with V thematrixwithcolumnvectorstherighteigenvectorsof P,and D adiagonalmatrixofeigenvalues V = 11 1 1/θ 2 , D = 10 0 θ 2 , V 1

Here θ + θ 2 = 1andthestationarydistributionis π (0) = 1/ (1 + θ 2 ), π (1) = θ 2 / (1 + θ 2 ).Thus

P0 { ˜ Xi = 0}= Pi (0, 0) = (VDiV 1 )00 = π (0) + O ( θ 2i ), P1 {Xi = 0}= π (0) + O ( θ 2i ).

Since Pa { ˜ Xi = ˜ Xi+1 =···= ˜ Xi+l 1 = 1}= Pa { ˜ Xi = 0}Pl 1 (0, 0),foreither startingstate a,foranystartingdistribution σ , Pσ ( ˜ Xi =···= ˜ Xi+l 1 = 1) = π (0)Pl 1 (0, 0) + O θ 2i Pl 1 (0, 0) . (1.3)

Fromthis, Pσ ˜ Mn ≥ (1 + )l = Pσ n l i=0 [0-runfrom i ≥ (1 + )l ] ≤ (n l )π (0)P l (1+ ) (0, 0) + O P l (1+ ) (0, 0) .

Fromthechoiceof l , P l (1+ ) (0, 0) = O (1/n1+ ),sotheright-handsidetends to0.

Toshowthat ˜ Mn /l ≥ (1 )withhighprobability,split[n]intodisjoint blocksoflength l (1 ) .Let ˜ Yi be1or0asthe ithblockisall0’sor not.Let W = n/l (1 ) i=1 ˜ Yi .Thesecondmomentmethodwillbeusedtoshow P{W > 0}→ 1.From(1.3),with l replacedby l (1 ) ,

Since ˜ Yi arebinary,theasymptoticsofthefirstsumin(1.5)areasin(1.4).

UsingtheMarkovproperty, Pσ ( ˜ Yi = ˜ Y j = 1) = Pσ ( ˜ Yi = 1)P0 ( ˜ Y j i = 1).The termsmaybeboundedusing(1.3)andthesecondsumisoforder n/n2(1 ) . Itfollowsthatthevarianceof W isofthesameorderasthemean,soaChebyshevboundshows

ThisprovesProposition1.6with Mn replacedby Mn .Thetransferofthelimit theorembackto Mn isroutinefromProposition1.4.

Remark. Morerefinedlimitingbehaviorof Mn willsurelybecoloredbythe nonexistenceoflimitingbehaviorassociatedwiththemaximumofdiscreterandomvariables.See[3,13].

IcannotleavethistopicwithoutremarkingonsomeamazingformulascommunicatedtomebyRichardStanley.Throughout,let Xi (x )bethe ithbinary digitofauniformlychosenpointin Fn .Definearandomvariable

Stanley(inpersonalcorrespondence)shows

In(1.6), f ( λ )isthedimensionoftheirreduciblerepresentationofthesymmetricgroup Sn correspondingtothepartition λ.Itiswellknown[11,pp.62–64] that f ( λ )equalsthenumberofinvolutionsin Sn+1 and f 2 ( λ ) = (n + 1)!. Theformulas(1.6)aresufficientlysurprisingthatanumericalcheckseems calledfor.Consider n = 3; S4 has10involutionsand24elements.For Wn (x ) theproductovertheemptysetis1:

x 000100010001101 Sum W3 (x )

)

Theasymptoticsof f ( λ )arewellknown[11,p.64].Thisgives

Fromthis,weseethat Wn isconcentratedarounditsmean.Proposition1.3and easiercalculationsshowthat Ln = log(Wn )hasalimitingnormaldistribution, so Wn islognormalinthelimit.

Asomewhatcontrivedsetofstepsleadingtoconsiderationof Wn maybe constructedasfollows.Supposeonewantedtoconsiderarandomsquare-free numberwithfactorsatmost x.Onenaturalwaytodothisconsiderstheuniform distribution.Anothernaturalapproachistoconsider 1 ,..., n independent, 0/1randomvariableswith P ( i = 1) = P ( i = 0) = 1/2 anddefine

with2 = p1 < p2 < ··· < pn thedistinctprimes.Aneasier(butquitesimilar) problemconsiders

Thishas

Soagain W n isconcentratedaboutitsmeanandlog W n isasymptoticallynormal.Therandomvariable Wn istheFibonacciversionof W n .(Okay,okay;I saiditwascontrived.)

Stanley’smotivationcomesfromhistheoryofdifferentialposets.In[14, Problem8],Stanleyconstructedasequenceofsemisimplealgebras An of dimension(n + 1)!whoseirreduciblerepresentationshavedegree Wn (x )for x ∈ Fn .Thusthenumberofirreduciblerepresentationsis Fn+2 .Theexistence of A1 ⊆ A2 ⊆ A3 ⊆ (withnicerestrictionproperties)isnothardtoshow by“generalnonsense.”Ausefulsetofgeneratorsandrelationswasfoundby

Okada[12].Thishasspawnedahostofinterestingdevelopmentsthatmaybe foundbyfollowingthecitationstoOkada’spaper.

Thetensionbetweenrecreationalmathand“realmath”isevidentthroughout theFibonacciworld.Asapartingshot,Iofferthefollowing:144isaFibonacci numberandit’salsoaperfectsquare(uh-oh).Also,8isaFibonaccinumber thatisacube(uh-oh).Arethereanyothers?No!Bugeaudetal.[1]provedthat 1, 8, 144aretheonlyFibonaccinumbersthatarepowers.Theirproofmakes realuseofthefullmachineofmodernnumbertheory.

Thereareothercodingsof Fn ;seehttps://oeis.org/A000045attheOn-Line EncyclopediaofIntegerSequences.Also,in[15],parts(b),(c),and(d)ofExercise1.35areaboutcompositionswithspecifiedparts.In[16],part(a)ofExercise7.66hasacuteproof.TherearealsoLucasnumbers;Idon’tknowany codingsforthem.Someofthesesuggestfreshquestions.Fortunately,Icancall Ron.

Acknowledgments

ThankstoSouravChatterjee,AngelaHicks,SusanHolmes,Kannan Soundararajan,RichardStanley,andmostofalltoRonGraham,forhelpwith thiswork.

References

1.Bugeaud,Y.,Mignotte,M.,andSiksek,S.Classicalandmodularapproachesto exponentialDiophantineequations.I.FibonacciandLucasperfectpowers. Ann. Math.(2) 163 (2006)969–1018.

2.Chung,F.andGraham,R.Onthediscrepancyofcircularsequencesofreals. J. NumberTheory 164 (2016)52–65.

3.D’Aristotile,A.,Diaconis,P.,andFreedman,D.Onmergingofprobabilities. SankhyaSer.A 50 (1988)363–380.

4.Diaconis,P.,andGraham,R. MagicalMathematics:TheMathematicalIdeasThat AnimateGreatMagicTricks.PrincetonUniversityPress,Princeton,NJ,2012.

5.Diaconis,P.,Graham,R.,andHolmes,S.P.Statisticalproblemsinvolvingpermutationswithrestrictedpositions.In StateoftheArtinProbabilityandStatistics(Leiden,1999).Vol.36of IMSLectureNotesMonogr.Ser. Inst.Math.Statist., Beachwood,OH,2001,195–222.

6.Erdös,P.Onintegersoftheform2k + p andsomerelatedproblems. Sum.Bras. Math. II (1950)113–123.

7.Graham,R.L.AFibonacci-likesequenceofcompositenumbers. Math.Mag. 37 (1964)322–324.

8.Graham,R.L.,Knuth,D.E.,andPatashnik,O. ConcreteMathematics:AFoundationforComputerScience,2nded.Addison-WesleyReading,MA,1994.

9.Hough,B.Solutionoftheminimummodulusproblemforcoveringsystems. Ann. Math.(2) 181 (2015)361–382.

10.Kannan,R.,Mahoney,M.W.,andMontenegro,R.RapidmixingofseveralMarkov chainsforahard-coremodel.In AlgorithmsandComputation.Vol.2906ofLecture NotesinComputerScience,Vol.2906,663–675.Springer,Berlin,2003.

11.Knuth,D.E. TheArtofComputerProgramming. Vol.3,2nded.Addison-Wesley, Reading,MA,1998.

12.Okada,S.AlgebrasassociatedtotheYoung–Fibonaccilattice. Trans.Am.Math. Soc., 346 (1994)549–568.

13.Révész,P.Strongtheoremsoncointossing.In ProceedingsoftheInternational CongressofMathematicians(Helsinki,1978).Acad.Sci.Fennica,Helsinki,749–754,1980.

14.Stanley,R.P.Differentialposets. J.Amer.Math.Soc. 1 (1988)919–961.

15.Stanley,R.P. EnumerativeCombinatorics. Vol.1.CambridgeStudiesinAdvanced Mathematics,Vol.49,2nded.CambridgeUniversityPress,Cambridge,2012.

16.Stanley,R.P. EnumerativeCombinatorics. Vol.2.CambridgeStudiesinAdvanced Mathematics,Vol.62.CambridgeUniversityPress,Cambridge,1999.

2

OntheNumberofONCellsin CellularAutomata

Abstract

Ifacellularautomaton(CA)isstartedwithasingleONcell,howmanycells willbeONafter n generations?Forcertain“odd-rule”CAs,includingRule 150,Rule614,andFredkin’sReplicator,theanswercanbefoundbyusingthe combinationofanewtransformationofsequences,therunlengthtransform, andsomedelicatescissorcuts.SeveralotherCAsarealsodiscussed,although theanalysisbecomesmoredifficultasthepatternsbecomemoreintricate.

2.1Introduction

Whenconfrontedwithanumbersequence,thefirstthingistotrytoconjecture aruleorformula,andthen(thehardpart)provethattheformulaiscorrect.This chapterhaditsorigininthestudyofonesuchsequence,1,8,8,24,8,64,24, 112,8,64,64,192, (A1602391 ),althoughseveralsimilarsequenceswill alsobediscussed.

Thesesequencesarisefromstudyinghowactivityspreadsincellular automata(forbackgroundsee[2,5,8,11,14,17,20,21,23,24,26]).Ifwe startwithasingleONcell,howmanycellswillbeONafter n generations?The precedingsequencearisesfromtheCAknownasFredkin’sReplicator[13].In 2014,Hrothgarsenttheauthoramanuscript[10]studyingthisCA,andconjecturedthatthesequencesatisfiedacertainrecurrence.Oneofthegoalsofthe presentchapteristoprovethatthisconjectureiscorrect;see(2.31).

In§2.2wediscussageneralclass(the“odd-rule”CAs)towhichFredkin’sReplicatorbelongs,andin§2.3weintroduceanoperationonnumber sequences(the“runlengthtransform”)thathelpsinunderstandingtheresulting

1 Six-digitnumbersprefixedbyArefertoentriesin[16].

sequences.Fredkin’sReplicator,whichisbasedontheMooreneighborhood, isthesubjectof§2.4,and§2.5analyzesanotherodd-ruleCA,basedonthe vonNeumannneighborhoodwithacentercell.AlthoughthesetwoCAsare similar,differenttechniquesarerequiredforestablishingtherecurrences.Both proofsinvolvemakingscissorcutstodissecttheconfigurationofONcellsinto recognizablepieces.

§2.6discussessomeotherCAsinone,two,andthreedimensionswhereitis possibletofindaformula,andsomeforwhichnoformulaisknownatpresent. Indimensionone,StephenWolfram’swell-knownlist[17,24,26]of256differentCAsbasedonathree-celledneighborhoodgivesrisetojustseveninteresting sequences(§2.6.1).Othertwo-dimensionalCAsarediscussedin§§2.6.2,2.6.3, andthethree-dimensionalanalogofFredkin’sReplicatorin§2.6.4.Thefinal section(§2.7)givessomeadditionalpropertiesoftherunlengthtransform.For manyfurtherexamplesofcellularautomatasequences,see[2]and[16](the indexto[16]listsmorethan200suchsequences).

2.2Odd-RuleCAs

Weconsidercellularautomatawhosecellsforma d -dimensionalcubiclattice Zd ,where d is1,2,or3.EachcelliseitherONorOFF,andanONcellwith centeratthelatticepoint u = (u1 , u2 ,..., ud ) ∈ Zd willbeidentifiedwiththe monomial xu = x u1 1 x u2 2 ··· xud d ,whichweregardasanelementoftheringof Laurentpolynomials R = GF(2)[x1 , x 1 1 ,..., xd , x 1 d ]withmod2coefficients. ThestateoftheCAisspecifiedbygivingtheformalsum S ofallitsONcells. AslongasonlyfinitelymanycellsareON, S isindeedapolynomialinthevariables xi and x 1 i ,andisthereforeanelementof R.Wewrite u ∈ S toindicate that u isON,thatis,that xu isamonomialin S. Inmostofthischapterwefocusonwhatmaybecalled“odd-rule”CAs.An odd-ruleCAisdefinedbyspecifyinganeighborhoodofthecellattheorigin, givenbyanelement F ∈ R listingthecellsintheneighborhood.Atypical exampleistheMooreneighborhoodin Z2 ,whichconsistsoftheeightcells surroundingthecellattheorigininthesquaregrid(seeFig.2.1(ix)),andis specifiedby F : = 1 xy + 1 y + x y + 1 x + x + y x + y + xy

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