Loomis-WhitneyInequalityPickm=n,c1=...cn=1n−1,andBi:Rn→Rn−1bethepro-jectionontotheithcoordinatehyperplane(meaningBi(x1,...,xn)=(x1,...,xi−1,xi+1,...,xn)),fori=1,...,n.Again,Condition1isstraightforwardtocheck.Fori=1,...,n,takeanyfi:Rn−1→R≥0suchthatRx∈Rn−1fi(x)n−1dx<∞.Then,Inequality2appliedtothefunctionsfn−1isimplifiestotheLoomis-Whitneyinequality:Zx∈RnmYi=1fi(Bi(x))dx≤mYi=1kfikn−1.Weremarkthatthisi* D E M O *usversionof)aspecialcaseofShearer’sLemma,see7.Shearer’sLemmaitselfcanberecoveredthroughdifferentchoicesof{Bi}and{ci}.ThisexampleillustratestwoimportantfeaturesofTheorem1.First,weprovestatementsaboutacollection{f1,...,fn}ofmanyf* D E M O *steadofjustm=2,asinHölder).Second,wecanchoosetheprojections{Bi}mi=1toreplaceanintegraloverRnwithintegralsoverRni,thelat* D E M O *aybeeasiertoevaluateinapplications.Moreover,the{Bi}and{ci}donothavetoallbethesame.TogetabetterunderstandingofwhattheLoomis-Whitneyinequalityissaying,letS⊂Rnbeameasurablesetandtakefi=I1/(n−1)Bi(S)tobetheindicatorfunction(raisedtothe1/(n−1)power)fortheprojectionofSontotheithcoordinateplane.Notethatifx∈SthenQni=1fi(Bi(x))=1.Thatis,ifapointisinthesetthenallofits(n−1)-dimensi* D E M O *ionsareinthe(n−1)-dimensionalprojectionsoftheset.Thismeans,thattheleft-handsideoftheLoomis-Whitneyinequalityisanupperboundonvol(S),wherevol(·)denotesthevolumeofasetinRn.Alongthesamevein,consideringeachofthetermsontheright-handside,wehavethatkfikn−1=#Zx∈Rn−1IBi(S)$1/(n−1)=(vol(Bi(S)))1/(n−1).Substituting,theLoomis-Whitneyinequalitythenstatesthat:vol(S)≤nYi=1vol(Bi(S))1/(d−1).Inotherwords,weobtainanupperboundonthevolumeofasetintermsofthevolumesofitslower-dimensionalprojections.Forexample,whenn=2,thissaysthattheareaofasetisatmostitslength(alongtheverticaly-axis)timesitswidth(alongthehorizontalx-axis).Theaboveinequalitygeneralizesthistohigherdimensions.ThisexampleillustrateshowTheorem1isoftenapplied;thefunctionsfi◦Biaretakentobeindicatorfunctionsoflowerdimensionalprojectionsaset,sothattheleft-handsideofInequality2representsthev* D E M O *etandtheright-handsideinvolvesvolumesofthelower-dimensionalprojections.SeethelecturenotesofBall[B+97]forfurtherapplicationsofthistechniqueinconvexgeometry.Application:CountingtrianglesinlargerandomgraphsWenowpresentanotherspecialcaseoftheBLinequalitythatwasrecentlyusedbyLubetzyandZhao[LZ15]toanalyzethenumberoftrianglesappearinginlargerandomgraphs.Specifically,oneofthetechnicallemmastheyprovedstatesthatiff:[0,1]2→[0,1]isamea* D E M O *tionsuchthatf(x,y)=f(y,x),then:%%%%%Z(x,y,z)∈[0,1]3f(x,y)f(y,z)f(z,x)dxdydz%%%%%≤"Z(x,y)∈[0,1]2f(x,y)2dxdy!3/2.(3)2 Roughlyspeaking,thetripleofpairs(x,y),(y,z),(z,x)correspondstoatriangleinagraphandthequantityontheleft-handsideof3representsthedensityoftrianglesinacertainrandomgraphassociatedwiththefunctionf.Thisinequalityallowsonetoestimatethenumberoftrianglesinthisgraphbasedonthenumberofedges,whichisasimplerquantitytounderstandinthesettingof[LZ15].Tosolvetheproblemofcountinglargersubgraphsthantriangles,theyderiveageneralizedversionofin-equality3.However,itisstraightforwardtoseethatinequality3isjustaspecialcaseoftheLoomis-WhitneyinequalityandthegeneralizedversionisjustanextensionofHölder’sInequality,whichisagainaspecialcaseofTheorem1.1.1PastworkontheBrascamp-LiebinequalitiesInthissection,weprovideabriefsummaryofthehistoryoftheBLinequality.Originallyproveninvariouss* D E M O *byLieb[Lie02],Brascamp,LiebandLuttinger[BLL74]andBrascampandLieb[BL76],thegeneralformoftheBL-inequalitygiveninTheorem1isduet* D E M O *l.[BCCT08].Inequalitiesofthistypehaveapplicationinmathematicalphysics[CLL05],convexgeometry[B+97]andbeyond.WebrieflymentionaninterestingconnectionbetweentheBLinequalityandtheoreticalcomputerscience.Asdiscussedpreviously,theconstantDinInequality2wascharacterizedin[BCCT08]asthesolutionofanoptimizationproblem.Recently,Gargetal.[GGOW18]studiedthealgorithmictaskofcomputingD,giventheprojections{Bi}mi=1andconstants{ci}mi=1.AlthoughallspecialcasesofTheorem1consideredinthisreportsatisfytheinequalitywithD=1,untiltheworkofGargetal.,itwasnotknownhowtocomputeDingeneralinpolynomialtime.2SubadditivityofentropyWebeginbyintroducingthebasicsubadditivityofentropyinequalityandthenstatingaslightgeneralizationofit(Han’sInequality(5)),yieldingaconnectiontoShearer’sLemma(7).Weendwithageneralstatementaboutthesubadditivityofentropy(8).Theorem2(BasicSubadditivityofEntropy).ForrandomvariablesX1,..,Xn,weh* D E M O *wing:H(X1,..,Xn)≤nXi=1H(Xi)(4)Thisstatementisasimpleresultthatcanbederivedfromthechainruleofentropyandthefactthatcondi-t* D E M O *lydescreaseentropy.Equalityin4holdsiffXi⊥Xj,∀i,j∈[n].Thisstatementhintsataconnectionbetweenaglobalquantity(JointEntropy)andalocalquantity(projectedentropies).Weno* D E M O *Inequality(5),whichisaslightgeneralizationofthestatementaboveandisthekeyingredientinthespecialcaseofaversionofShearer’sLemma(7).Han´sInequalityHan’sinequalityallowsustocontrolthejointentropyofasystemviathemarginalentropies:H(X1,..,Xn)≤1n−1nXi=1H(X1,..,Xi−1,Xi+1,..,Xn)(5)•Han’sinequalitycanbeprovedbyrewritingthejointentropyasaconditionalsumofentropies(forsomei∈[n]),droppingconditioningonallcoordinates>i,repeatingthis∀i,andthensummingupthetermsandapplyingthechain-ruleforentropy.3 •Inthespiritofgeneralizingfurther,Han’sinequalitycanbeappliedtorelativeentropiesaswell.GivenafinitealphabetX,aproductdistributionP=nYi=1pionXn,andajointdistributionQonXn,itholdsthat:DKL(QkP)≥1n−1nXi=1DKL(Q(i)kP(i))(6)whereP(i),Q(i)denotethemarginaldistributionsforP,Qrespectively.Shearer’sLemma&Han’sInequalityS* D E M O *maisastatementthatallowsustoboundthevol-umeofadiscretehigh-dimensionalobjectbylookingatitslow-dimensionalprojections.Theorem3(Shearer’sLemma).Givensomen,k,N∈Z+wherek≤n,anobjectF=(f1,..,fn)⊆[N]n,andthek-dimensionalprojectionsFS={(fi1,..,fik)|ij∈S,S⊂[n]},thefollowinginequalityholds:|F|(n−1k−1)≤YS⊂[n],|S|=k|FS|(7)IfwereformulateShearer’sLemmabyreplacingthevolumeterms|·|in7bytheirentropiccounterparts,thenwerecoverHan’sInequality5whenk=n−1,andBasicSubadditivityofEntropy4whenk=1.Thishintsataconnectionbetweensubadditivityofentropyandproblemsingeometrythatinvolveestimatinghigh-dimensionalvolumesbylookingatlow-dimensionalprojections.SincethestatementoftheBrascamp-Liebinequality(2)canbeinuitivelyinterpretedastryingtoboundthevolumeofahigh-dimensionalcontin-uousobjectviaitslow-dimensionalprojections,itisnaturaltoquestionwhethersubadditivityofentropy(initsgeneralform)isrelatedto2.Ageneralformofsubadditivityofentropy,whichsubsumes4,5and6is:Theorem4(SubadditivityofEntropy,[BLM13]).GivenindependentrandomvariablesX1,..,XntakingvaluesinacountablesetXwiththeproductdistribution:P=nYi=1piandZ=f:Xn→[0,∞)apositive,boundedrandomvariable,thefollowinginequalityholds:Ent[Z]≤EP[nXi=1Ent(i)[Z]]1(8)Thisequationhasasimilarskeletalstructureto4,andbasicallyassertsthatthejointentropyofthesystemis* D E M O *abovebytheaverageofthesumofthemarginalentropies.ThegeneralizedEnt[Z]termallowsustocontrolhighermoments,formingthebasisoftheEntropyMethodforprovingconcentrationinequalities.21Ent[Z]isageneralizationofentropyandEnt(i)[Z]isamarginalofthegeneraliz* D E M O *neEnt[Z]=E[Zlog(Z)]−E[Z]log(E[Z]),andthecorrespondingmarginal:Ent(i)[Z]=E(i)[Zlog(Z)]−E(i)[Z]log(E(i)[Z]).Referto[BLM13]foracompletetreatment.2Toseehow8cantigthenaconcentratio* D E M O *,refertoChapter-4of[BLM13].Fortheex* D E M O *badditivitytomoregeneralentropies,refertoChapter-14in[BLM13].4 3DualityofsubadditivityofentropyandBrascamp-LiebInequalityWhilethereareseveralproofsofTheorem1,thefirstofwhichwasduetoBennettetal.[BCCT08],wenow* D E M O *ternativeapproachduetoCarlenandCordero-Erausquin[CCE09].Amongotherthings,theyshowthatInequality2isdual(inaprecisesensetobeexplainedbelow)toacontinuousversionof8(see9).ThisexactlyquantifiesandgeneralizestheconnectionwebrieflysawinSection2.3.1StatementofthemainresultWenowgiveoneofthemainresultsofCarlenandCordero-Erausquin,whichestablishesadualitybetweentheBLinequalityandsubadditivityofentropy.Foraprobabilitydensityfunctionf:Rn→R≥0withRx∈Rnf(x)dx=1,wecandefinetheentropyasS(f)=Zx∈Rnf(x)logf(x)dx.Infact,thisisactuallythenegativeentropy,butitwillbesimplertoworkwiththisversion.So,aswewillsee,"subadditivity"ofthisdefinitionofentropyreallymeanssuperadditivity.ForaprojectionBi:Rn→Rni,weusefBitodenotethemarginalprobabilitydensityfunctionoffunder* D E M O *onBi3.Withthesedefinitions,wecanstatethemainresult.Theorem5(Theorem2.1of[CCE09]).Let{Bi}mi=1and{ci}mi=1beasinTheorem1.Thenthefollowingstatementsareequivalent:1.(BLinequality)Foranycollection{fi}mi=1ofnon-negativefunctionsfi:Rni→R≥0suchthatRx∈Rni|fi(x)|dx<∞,Inequality2holds.2.(Subadditivityofentropy)Foranyprobabilitydensityfunctionf:Rn→R≥0withRx∈Rnf(x)dx=1andS(f)<∞,thefollowinginequalityholds:mXi=1ciS(fBi)≤S(f)+logD.(9)HavingstatedTheorem5,wecanseethatbothInequalities2and9arestatementsthatrelateintegralsofafunctiontointegralsofitsmarginals.Moreover,notethatTheorem5preservestheconstantDwhenmovingbetween2and9.CarlenandCordero-Erausquin[CCE09]exploitthisfacttocharacterizethefunctionswhichmaketheBLinequalitytight.3.2DualrelationshipWecanconnectentropyandtheBLinequalitythroughthefollowingformulafrom[CCE09].Theorem6.LetfbeaprobabilitydensityfunctiononRnandφ:Rn→RbeanyfunctionsuchthatRx∈Rneφ(x)dx<∞.Then,thefollowingholds:log!Zx∈Rneφ(x)dx"≥hf,φi−S(f).(10)Thequantityo* D E M O *ndsideofInequality10isknowninothercontextsasthelogarithmofthemomentgeneratingfunction(orthecumulantgeneratingfunction)ofφ.Inequality10simplysaysthatthedual4ofentropyS(f)isthelogarithmofthemomentgeneratingfunction.3ThismeansthatiftherandomvariableXisdistributedaccordingtof,thenBiXisdistributedaccordingtofBi.4Formally,thisiscalledFenchel-Legendreduality,awell-studiednotiononitsownright.See[Roc15]forareference.5 Proof.Ourstartingpointistherelativeentropybetweenφandf,whichcanbesimplifiedtogettheright-handsideoftheinequalitywewanttoprove:Zx∈Rnf(x)log"eφ(x)f(x)!dx=hf,φi−S(f),wherewehaveusedtheinnerproducthf,φi=Rx∈Rnf(x)g(x)dx.BytheconcavityofthelogarithmandJensen’sinequality,wecanupperboundtherelativeentropyas:Zx∈Rnf(x)log"eφ(x)f(x)!dx≤log"Zx∈Rnf(x)eφ(x)f(x)dx!=log#Zx∈Rneφ(x)dx$.Combiningthepreviousinequalitywiththeoriginalequalitygivestheresult.3.3ProofofTheorem5TheproofstrategyforTheorem5issimple.WewillassociatetheBLinequalitywiththeleft-handsideofFormula10andInequality9withtheright-handside,allowingustopassbackandforthbetweentwo.Proof.Forthefirstdirection,assumethatInequality2holdsandletfbeanyprobabilitydensityfunctiononRnwithS(f)<∞.Instantiatingthedualityformula10withφ=Pmi=1cilogfBi◦Biandrearranging,wehavethat:S(f)≥mXi=1ci#Zx∈RnifBi(x)logfBi(x)dx$−log"Zx∈RnmYi=1fBi(Bix)ci!.Ontheright-handsideoftheaboveinequality,weapplytheBLinequalitytocontroltheintegralofproductssothatwegetZx∈RnmYi=1fBi(Bix)ci≤DmYi=1#Zx∈RnifBi(x)dx$ci=D,whereweusedthefactthateachfBiisadensityfunctionwhichintegratesto1.Substitutingthisintotheaboveinequality,wehave:S(f)≥mXi=1ci#Zx∈RnifBi(x)logfBi(x)dx$−logD=mXi=1ciS(fBi)−logD,completingthefirstdirectionoftheproof.Thereversedirectionissimilar,soweomitthedetails.Theproofillustratesthatthelogarithmofthemomentgeneratingfunctionmakesiteasiertoworkwithmarginals,sincetheexponentialofasumfactorsintoaproductofexponentials.4ConclusionInthisreport,wesawthattheBLinequalityisageneraltoolforboundingintegralsofproductsoffunctionsbyproductsofintegrals,whichariseinvariousapplications.ThemaintakeawayofTheorem5isthatwecannowproveBL-typeinequalitiesbymakinguseoftoolsfrominformationtheorytoprovethecorrespondingsubadditivityofentropyinequalities.Insomecases,thelattertaskissignificantlyeasier(asdemonstratedbyCarlenandCordero-Erausquin[CCE09]).Asasimpleexampleofthis,instantiateTheorem5with{Bi}ni=1astheprojectionsontothencoordinatehyperplanes.ThenInequality9becomesHan’sInequality5andInequality2becomestheLoomis-WhitneyInequality.6 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