ON THE MAILLET–BAKER CONTINUED FRACTIONS

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ON THE MAILLET–BAKER CONTINUED FRACTIONS BORIS ADAMCZEWSKI, YANN BUGEAUD Abstract. We use the Schmidt Subspace Theorem to establish the transcendence of a class of quasi-periodic continued fractions. This im- proves earlier works of Maillet and of A. Baker. We also improve an old result of Davenport and Roth on the rate of increase of the denominators of the convergents to any real algebraic number. 1. Introduction A central question in Diophantine approximation is concerned with how algebraic numbers can be approximated by rationals. This problem is inti- mately connected with the behaviour of their continued fraction expansion. In particular, it is widely believed that the continued fraction expansion of any irrational algebraic number ? either is eventually periodic (and we know that this is the case if, and only if, ? is a quadratic irrational), or it contains arbitrarily large partial quotients. Apparently, this problem was first considered by Khintchine in [11] (we also refer the reader to [3, 21, 22] for surveys including a discussion on this subject). Some speculations about the randomness of the continued fraction expansion of algebraic numbers of degree at least three have later been made by Lang [12]. However, one shall admit that our knowledge on this topic is up to now very limited. A first step consists in providing explicit examples of transcendental con- tinued fractions.

  • eventually periodic

  • his

  • fraction expansion

  • algebraic numbers

  • partial quotients

  • positive integers


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ONTHEMAILLET–BAKERCONTINUEDFRACTIONSBORISADAMCZEWSKI,YANNBUGEAUDAbstract.WeusetheSchmidtSubspaceTheoremtoestablishthetranscendenceofaclassofquasi-periodiccontinuedfractions.Thisim-provesearlierworksofMailletandofA.Baker.WealsoimproveanoldresultofDavenportandRothontherateofincreaseofthedenominatorsoftheconvergentstoanyrealalgebraicnumber.1.IntroductionAcentralquestioninDiophantineapproximationisconcernedwithhowalgebraicnumberscanbeapproximatedbyrationals.Thisproblemisinti-matelyconnectedwiththebehaviouroftheircontinuedfractionexpansion.Inparticular,itiswidelybelievedthatthecontinuedfractionexpansionofanyirrationalalgebraicnumberξeitheriseventuallyperiodic(andweknowthatthisisthecaseif,andonlyif,ξisaquadraticirrational),oritcontainsarbitrarilylargepartialquotients.Apparently,thisproblemwasfirstconsideredbyKhintchinein[11](wealsoreferthereaderto[3,21,22]forsurveysincludingadiscussiononthissubject).SomespeculationsabouttherandomnessofthecontinuedfractionexpansionofalgebraicnumbersofdegreeatleastthreehavelaterbeenmadebyLang[12].However,oneshalladmitthatourknowledgeonthistopicisuptonowverylimited.Afirststepconsistsinprovidingexplicitexamplesoftranscendentalcon-tinuedfractions.ThefirstresultofthistypegoesbacktothepioneeringworkofLiouville[14],whoconstructedtranscendentalrealnumberswithaveryfastgrowingsequenceofpartialquotients.Subsequently,variousau-thorsuseddeepertranscendencecriteriafromDiophantineapproximationtoconstructotherclassesoftranscendentalcontinuedfractions.OfparticularinterestistheworkofMaillet[15](seealsoSection34ofPerron[17]),whowasthefirsttogiveexplicitexamplesoftranscendentalcontinuedfractionswithboundedpartialquotients.HisworkhaslaterbeencarriedonbyA.Baker[4,5].Moreprecisely,Mailletprovedthatifa=(an)n0isanon-eventuallype-riodicsequenceofpositiveintegers,andifthereareinfinitelymanypositiveintegersnsuchthatan=an+1=...=an+λ(n)1,thentherealnumberξ=[a0;a1,a2,∙∙∙]istranscendental,assoonasλ(n)islargerthanacertainfunctionofthedenominatorofthen-thconvergenttoξ.Actually,theresultofMailletismoregeneralandalsoincludesthecaseofrepetitionsofblocksofconsecutivepartialquotients(seeSection2).HisproofisbasedonageneralformoftheLiouvilleinequalitywhich1