Moment

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MomentsumsassociatedwithbinarylinearformsP.ShiuAbstractAtransparentargumentisusedtostudythestructureofthesetofnumbersrepresentableasax+by,withx,y≥0.Momentsumsofrepresentablenumbersare

evaluatedintermsofBernoullinumbers.

1.IntroductionLetaandbbecoprimeintegersexceeding1,anddenotebySthesetofnon-negativeintegersm

ax+by=m(1.1)issolublewithx,y≥0.Thelargestnumbermforwhich(1.1)isnotsolubleisknowntobetheoddnumber

M=ab−a−b;(1.2)see,forexample,[1,Theorem1.8.2],butnotmuchhasbeensaidconcerningthosemasimplecriterionform∈Swhichisthenusedtoevaluatethemomentsums

Sr=󰀈m∈Smr,r=0,1,2,....(1.3)

LetBr=Br(0),whereBr(x)aretheBernoullipolynomials,andwrite

p(r;a,b)=1ab󰀈0≤k≤r+2h+k=r+2ahbkBhBkh!k!,r=−2,−1,0,1,....(1.4)

WeshallprovethefollowingTheorem.Let0≤m≤M.Thenm∈SifandonlyifM−m∈S.Also,forr=0,1,2,...,Srr!=Br+1(ab)(r+1)!+p(r;a,b)−󰀈0≤k≤r+2h+k=r+2(ab)hp(k−2;a,b)

h!.(1.5)

1991MathematicsSubjectClassification:11D04,11L99.–1–Thus,S0=

ab+a+b−1

2,

S1=4a2b2+3ab(a+b)−a2−b2−9ab+1

12,

S2=ab󰀁3a2b2+2ab(a+b)−a2−b2−9ab+a+b+2

󰀂

12,

and,ingeneral,Srisapolynomialina,bwithleadingterm(ab)r+1/(r+2).In

anearlierversionofthepaper,althoughamethodofevaluatingSrwasgiven,thealgebrainvolvedwastediousunlessrissmall,andthegeneralformula(1.5)wasnotfound.Inowthanktheconscientiousrefereewhoofferedtheelegantproofofthelemmain§4whichidentifiesthegeneratingfunctionassociatedwiththemomentproblem.Myearlierlaboriousmethodinvolvesthe‘weightedexponentialsums’

Ψ(a,b;s,t)=󰀈0≤xxsψ󰀃axb󰀄t,(1.6)

where,asusual,ψ(λ)=λ−[λ]−12.Sinceψ(λ)isperiodicwithperiod1,Ψ(a,b;s,t)

dependsonaonlytotheextentthatitactuallydependsontheresidueclass(modb)towhichabelongs,andthereareinteresting‘reciprocityrelationships’forsuchsums.Forexample,intheabsenceofusefulexplicitexpressionsevenforΨ(a,b;1,1)andΨ(a,b;2,1),theidentities

aΨ(a,b;1,1)+bΨ(b,a;1,1)=a2+b2−3ab+1

12,(1.7)

a2Ψ(a,b;2,1)+b2Ψ(b,a;2,1)=ab(a2+b2−3ab+1)

12,(1.8)

areefficienttoolsfortheircomputationwhenaandbaregivenlargecoprimeintegers.Thus,insteadofhavingtosumoverbtermsin(1.6),theirvaluescanbecomputedbyapplying(1.7)and(1.8)inO󰀁log(a+b)󰀂operationsthroughthecorrespondingstepsoftheEuclideanalgorithmtodeliverGCD(a,b)=1.Thenumberofsolutionsto(1.1)isdenotedbyN(m;a,b)andtheargumentnormallyusedforitsdeterminationissketchedin§2.Ournewapproachisgivenin§3,andtheproofof(1.5)isgivenin§4.Weomittheproofsof(1.7)and(1.8),whichrelyonpartialsummations.

–2–2.AformulaforN(m;a,b)Thegeneralsolutionto(1.1)involvesaparameterwhoserealvaluesfornon-negativerealsolutionsx,yformanintervalwithlengthm/ab.Ifm≥abthenthisintervalcontainsaninteger,andsotheDiophantineequationhasanon-negativeintegersolutioninx,y.WhenMhaslengthlessthan1,butitstillcontainsaninteger.Infact,foranygivenvalueofm,Diophantineanalysiscanbeemployedtocounttheadmissibleintegervaluesoftheparameter,whichthenleadstotheformula

N(m;a,b)=mab+1−󰀋¯bma󰀌−󰀋¯amb

󰀌

,(2.1)

where{λ}denotesthefractionalpartoftherationalnumberλ,and¯a,¯bareintegerssatisfying¯aa≡1(modb),¯bb≡1(moda).(2.2)

AnothermethodoffindingaformulaforN(m;a,b)isbasedonthetheoryofrestrictedpartitions,inwhich(1.1)isconsideredasapartitionofmintothepartsaandb,andaformulacanthenbeobtainedbyresolvingtheassociatedgeneratingfunctionintopartialfractions.AnyformulaforN(m;a,b)willshowthatitsvalueiseither[m/ab]or[m/ab]+1,togetherwithacriterionwhichdeterminesitsvalue.Thus,for0≤mvalueofN(m;a,b)actsasacharacteristicfunctionforthesetS.In(2.1)thedeterminationofN(m;a,b)restswiththevalueofthesumoftwofractionalparts,whichcanbeusedtoidentifyindividualvaluesmwithN(m;a,b)=0,thatismofS0canbedealtwithbyevaluatingthesumsofthefractionalpartsin(2.1).

However,asweshallsee,ourargumentinthenextsectionismorenaturalandmuchsimpler.ItmaybeofinteresttomentionthatN(m;a,b)playsacentralroleinaposthumouspublication[2]inwhichL.K.Huamadea‘directattempt’onGoldbach’sconjecture.ByexpressingthenumberofrepresentationsofanevenintegerasasumoftwoprimesintermsofelementaryarithmeticfunctionsHuausedexponentialsumsatthemostbasicleveltoisolatewhatisexpectedtobetheprincipaltermforthesaidnumber.Thus,byapplying(2.1)andwithouttheuseoftheformidablemachineryoftheHardy-Littlewoodmethod,Huamanagedtorecovertheesotericsingularseriesassociatedwiththenotoriousconjecture.

–3–