; TeX output 1998.02.26:1651Ef4ff~L,K`yff cmdunh10NewYorkJournalofMathematicsffQऄff2 w5html:|{Ycmr8NewXYJorkJ.Math. html: 8html:-2@cmbx83A html:(1998)69{.!ffL Nff cmbx12AffNoteontheApproximationbyContinuedkFfractionsffunderanExtraCondition html:"N cmbx12KarmaDajaniT: html:andhtml:CorKraaik@amp[( html:BN8Zfcmcsc8Abstract.ֹInHthisnotethedistributionoftheapproÎximationco html: #- cmcsc10Contents K`y cmr101. html:IntroGduction html: x69 2. html:AUUNaturalExtensionofaSkewProGductbyJagerandLiardet html:170 3. html: b> cmmi10S-expansions html:76 html:References html:#79\khtml: html:!XQ cmr121.Intro`duction AZclassicalresultbyHurwitzstatesthatforeveryirrationalnumbGerxthereexist in nitelyUUmanypairsof(co-prime)integerspandq[ٲ,q">0,suchthatN􍍍Uu cmex10 U U U 4x8 !", cmsy10<$lplwfe (֍q F  F F F Z<<$ ϟ1wfe UX EPpUWPfeE5<$?1`wfe O (֍q[ٟrٓRcmr72#8Q:(html: html:1)InthepastcenturyagreatnumbGerofpapersappeared,aimedatreproving,re ningorZfgeneralizingHurwitz'result(html:1 html:).HerewementionatheorembyKoksma[html:Kok= html:=],whichUUinitselfwasare nementofaresultbyHartman[html:H html:].؍*"V cmbx10TheoremThtml: html:1.> (Koksma))': cmti10L}'etxbeanirrationalrealnumber,andletm<1;UPaandbb}'eintegers.%Thenforevery >0thereexistin nitelymanypairsofintegerspandq[,q">0,suchthat(h.  .  .  .  1arx8<$lplwfe (֍q F  F F F R<<$m^2|s(18+)wfe*SD E~PpMPfeE5<$4+12^wfe O (֍q[ٟr2C$andZpa(moGdm);UPq"b(moGdm):(html: html:2)nThec}'onstantPp >PfeE5}&isbestpossible,i.e.,itcannotbereplacedbyalargerone. :dff< O[ ReceivÎedXNovemb html:70r.F C cmbxti10Karma$DajaniandCorKri>aaikampNBलTheproGofofthistheorem,DandalsothatofrelatedresultsbyDescombGesand 6Poitou':[html:DPq html:q],0rrestsonthestrongapproximationpropGertiesoftheregularcontinued6fraction.BInӬthisnotewewilldetermineforalmostallhtml:^1|s html:xtheasymtoticdensityofthose6regularwcontinuedfractionconvergentsp 0ercmmi7nq~=qnofxsatisfying(html:2 html:).'T*obGemoreprecise,6letUUA(x;c;N)bGethecardinalityofthesetA(x;c;N),de nedas6L8dfQ8en;UP1nN;qnq~jqnx8pnq~j html:3)$6HereUUJKisJor}'danP'sarithmeticaltotientfunction,UUde nedby̵J9(m)q:=m2Y j'pjm(18<$L1lwfe s (֍pr2#);䍑6ल(theproGductistakenoveralltheprimespforwhichpjm)andF胲isadistribution6functionUU-theso-calledL}'enstracurveUU(seealso[html:BJW html:])-givenby>`Fc(zp)q=􍓫8 < :č SŴz&felog 2$m˵;{r0z7 K1K&fes2 ); 5 h cT1&felog 2$m˲(zw+8log42z+81); }+1}+&fes2"cz71:BलTheabGoveresult(andseveralothers)willbGeobtainedfromasuitablenatural 6extension=@ofaskewproGductwhichwasintroGducedandstudiedbyH.Jagerand6P*.fLiardet[html:JL c html: c].InthelastsectionwewillextendtheseresultstoS-expansions,6whichformaverylargeclassofcontinuedfractionexpansions.6Infactwewill6see,Ythat{theresultsobtainedbyJagerandLiardetin[html:JL c html: c]onthenumeratorsand6denominatorsUUofconvergentsUUholdforanyS-expansion.6ट )html: html: 92.ANaturalExtensionofaSkewPro`ductbyJagerandLiardet LetUUtheregularcontinuedUUfractionexpansion(RCF)ofx2[0;1)UUbGegivenby6yxq=[0;UPB1|s;:::UG;Bnq~;:::]:(html: html:4)oҍThisexpansionis niteifandonlyifxisrational.7Finitetruncationin(html:4 html:)yieldstheөsequenceofRCF-convergentsөpnq~=qn >=q[0;UPB1|s;:::UG;Bnq~];n1:өUnderlyingtheRCFUUistheopGeratorTO:q[0;1)![0;1),UUde nedbyٍ_Tc0:=0andqTxq:=<$1wfe (֍x"8b<$133wfe (֍xc;x6=0;2?wheremθbuǸcisthe o}'or(orentier)vqalueof.3LetbGetheso-calledGausskme}'asureonUU[0,1),i.e.,isaprobabilitymeasureon[0,1)withdensity<$ƙ1{wfeU (֍logT2<$O1"Rwfe (֍18+x(:$Itr\iswell-knownr\thatthedynamicalsystem([0;1)rnQ |;;Tc)r\isergoGdic,seee.g.[html:R-N1 html:1]. *ff< O[ -:1*html: html:AllXalmost~alp[lstatemenÎtswillboximation$byContinuedFractionsGhtml: html:71NBF*oram|ָ2Z ,ݤm2,letaG(m)bGethegroupof22amatriceswithentriesfrom 6Z=羵=mZ \oandUUdeterminant1,i.e.,39bG(m)q:=^ ^dN5 (R  )k3n^>2;UP z; ; 8;'2Z 2=mZ; ø8  =1^i]:3:6लInUU[html:JL c html: c]itwasshownthatoycard{G(m)q=^d2mJ9(m);Om3;mJ9(m)=6;Om=2:6लSettingq΍7An >:=q^d40#1415Bn1{^:ܗ;Mn:=qA1'|jAnq~;n1;6लoneUUhas,seee.g.[html:K html:],_Mn >:=q^d4pn1/&pn̵qn1/̾qn>^Ih;n1:h6लNoticemthatthewell-knownmrecursionrelationsforthepnq~'sandqn'satoncefollow6fromUUMn+1X=Mnq~An+1.BDenotingbyhm aHaarmeasureonG(m),%andsettingjG(m)j2:=cardrG(m),6H.UUJagerandP*.Liardet[html:JL c html: c]obtainedthefollowingtheorem.g6TheoremThtml: html:2.u(Jager&Liardet)L}'etm2Z 2;UPm2.fF;orܵx2(0;1),EletB1|s(x)=b yl133&fexJc6anddenotea(moGdm)by~feI0ga.Thentheskewpr}'oduct32u:=([0;1)8nQ KG(m);UP hm;L)sF;6ल(html: html:5)>6wher}'ethetransformationLisgivenby1čp7L(x;g[ٲ)q:=^<$ \1 gwfe (֍x$8B1|s(x);UPg^Yb0'01 b1bLщfeu/B1(x);k^COߟ^#;(x;g[ٲ)2;1Í6iser}'godic.BF;urthermor}'e,foralmostallx2[0;1)thesequenceofmatrices]nq7!^d4~fegpn1/&~fe y~gpn̟~fe$ßgqn1/̾~fe Ogqn>^Ih;n1;3:6isuniformlydistribute}'doverG(m).gBलF*rom!Theoremhtml:2 html:andtheErgoGdicTheorem,TJagerandLiardet[html:JL c html: c]wereable6toƐdrawanumbGerofcorollaries,"someofwhichwerepreviouslyobtained(ina6completelyBdi erentway)byR.MoGeckel[html:M * html: *].kHerewementionthefollwingresult.6PropQositionThtml: html:1.(MoGeckel;Jager'&Liardet)DL}'etp;UPqwandmbethreeintegers,Tzsuch6thatm2and(p;q[;m)=1.Thenforalmostallxonehas39Xdlims3T>oN,!1<$p3IJ1n#şwfe  (֍Nxv#^ *n;UP1nN;^dĵpn\qn+B^*vq^d4pq4^%(moGdm)^?=<$W1Owfe (֍J9(m)!>:oBलF*urthereinterestingapplicationsofTheoremhtml:2 html:wereobtainedbyV.Noltein[html:No  html: ].BInkordertoshowthatforalmosteveryxthelimitin(html:3 html:)existsandequals6वFc(c)=J9(m),UUtheabGoveUUskewproGductisnotsucient.qT*oseethis,put32-n8=nq~(x):=qnjqnx8pnj;UPn0;6लand (Tnq~;Vn)q:=(Tcn x;<$qn1wfe$ß (֍:qny);UPn0:H$D|46html: html:72rKarma$DajaniandCorKri>aaikampN6लThenUUoneeasilyshows(seee.g.[html:K html:]),that퍍-n1=<$"Vnwfe'0 (֍18+Tnq~Vn0K; UQn >=<$̵Tnwfe'0 (֍18+Tnq~Vn:6ल(html: html:6)ō6Sinceqn1=qn8=[0;UPBnq~;:::;B1|s],Ӎweseethatn$depGendsonboth`thefuture'(i.e., 6वTnq~)1'and`thepast'(Vn)ofx.eInordertostudythese's,8cW.Bosma,H.Jagerand6F.mWiedijkusedin[html:BJW html:]anaturalextensionof([0;1)I,nQ 췵;UP;Tc),swhichmwas rst6studiedbyH.Nakqada,LS.ItoandS.T*anakain[html:NITUW html:UW],Lseealso[html:Na  html: ].Herewewill6studytheskewproGductofthisnaturalextensionwithG(m),Iandwewillshowthat6thisnewskewproGductisactuallythenaturalextensionoftheskewproGductused6byJagerandLiardet.9W*e rstrecallthede nitionofnaturalextension,oseealso6[html:R\r html:\r]UUor[html:Br  html: ].F6De nitionThtml: html:1.zL}'etTwFandSDbetwomeasurepreservingtransformationsof(X:;BM۵;)6and(Y9;CW;)r}'espectively.{ThetransformationSSissaidtobeafactormapofT#if6ther}'eexistsameasurablemap͙:q(X:;BM۵;)!(Y9;CW;)suchthatҖx(i)B˵j=8[ٟ1 M;(ii)'〵T*=Sm:6De nitionThtml: html:2.zAninvertible,me}'asurepreservingtransformationT@on(X:;BM۵;)is6saidܫtob}'ethenaturalextensionofthemeasurepreservingtransformationSp8on6ल(Y9;CW;)Fifther}'eexistsafactormap:*(X:;BM۵;)h!(Y;CW;)FsuchthatBw=6स_^1ln=0Tc^n ([ٟ^1 MCW)UP(mo}'d U0):BलW*eUUhavethefollowingtheorem.X6TheoremThtml: html:3.uL}'etڈmbeaninteger,1mG 2,andڈletB1|s(x)andLщfeu/B1(x)!Eb}'ede nedas6b}'efore.Let q:=[0;1)8nQ _[0;1],andlet~feg .pb}'etheprobabilitymeasureon ,with6densityCd(x;y[ٲ)q:=<$ 1wfeU (֍logT2<$-[1wfe(v (֍(18+xy)r2H;UP(x;y)2 :6Finally,letTQò:q 8G(m)! 8G(m)b}'ede nedbyhtml:^2|s html:2MT(x;y[;g)q:=^<$ \1 gwfe (֍x$8B1|s;<$ϋ1wfe (֍B1S+y#I;UPg^b0 1 n鍍b1b}fe ;B1.u1^5ѥ^;(x;y;g)2 8G(m):W6Thentheskewpr}'oduct&( 8G(m);UP~feg hm;UPT)6(html: html:7)HY6isthenatur}'alextensionoftheskewproductfrom(html:5 html:),andisthereforeergodic.6Remarks6टW^html: html: 1.Infact,theskewproGduct(html:7 html:)hasmixingpropertiesfarstrongerthanergod-icity*.4In1[html:L@ html:@],hLiardetshowedthat(;UP۸ hm;L)1isexact,handthereforeitfollows,UUseealso[html:Br  html: ],p.q39,that( 8G(m);UP~feg hm;UPT)UUisaK-system.html: html: 2.ThevPpro8jection~ L:q zڸG(m)! vPgivenby~n(x;y[;g):=(x;y)vPyieldstheafore-mentionednaturalextension( ;~feg.;Ny=)byNakqada-Ito-T*anakaof([0;1);;Tc).ClearlyUUNy=(x;y[ٲ)=(Tcx;UP(B1|s(x)8+y)^1 t)UUfor(x;y)2 .}ProQofTofTheoremhtml:3 html:.kmLetwյ":q }G(m)![0;1)}nQ QG(m)denotethepro8jection[ٲ(x;y;g):=(x;g[ٲ).qThenUUclearlyonehas Ðff< O[ -:2*html: html:WJeXwillsuppressthedepoximation$byContinuedFractionsGhtml: html:73N@(i)Oµ[ٸTk=qL.; =Rj(ii)O(~feg?f 8hm)[ٟ^1 =q hm ; UM:M(iii)O¸_n0T^n)([ٟ^1 M(B8F9))q=}feIB FS;ߍ6where3nBIistheBorel[ٲ-algebraon[0;1)nQ p{,j}feIB}is3ntheBorel-algebraon and6सFistheBorel[ٲ-algebraonG(m).bNoticethat(iii)isanimmediateconsequence6ofmERemarkhtml:2 html:withFk~thepGowermEsetofG(m).Thussatis estheconditionsof6De nition}]html:2 html:,_andthereforeTisthenaturalextensionofLasgivenin(html:5 html:).As6iswell-known,]seee.g.[html:R\r html:\r]or[html:Br  html: ],thenaturalextensionTYinheritsallmixing6propGerties#nfromS.SinceJagerandLiardetshowed#nthatSisergodic,Vtheresult6follows.?% msam10Í6LemmaThtml: html:1.muF;oralmosteveryx2[0;1)andeveryg"2G(m)these}'quencem-(Tn)(x;0;g[ٲ))ȟn0ȍ6isdistribute}'dover :&G(m)accordingtotheprobabilitymeasure~feg T :&hm,i.e.,for6everyVBor}'elsetD"in ZG(m),bfforValmosteveryx2[0;1)Vandforeveryg"2G(m)6onehasލp liml˴n!1<$^1Nwfe  (֍N#fn;UP1nN;Tn)(x;0;g[ٲ)2DGgq=~feg & 8hm(D):д6ProQof.[Notice,ZthatYifthesequence(Ny=^n껲(x;y[ٲ))n0ŲisdistributedoverY according6toUUthedensityd,i.e.,ifforeveryBorelsetBG onehas׍lim}شn!1<$p1`wfe  (֍N,#fn;UP1nN;Ny=n껲(x;y[ٲ)2Bqgq=~feg xF(B);46लthenUUitfollowsfrom~feg 8hm(BQfg[ٸg)q=<$ ~fegx(Bq)wfe۟ (֍jG(m)j$;ݍ6लwhere͵BPN߲ isaBorelsetandg2G(m),*thatforeveryg2G(m)thesequence6(T^n)(x;y[;g))n0GisUUdistributedoverUU 8G(m)accordingto~feg hm.BNextUUobserve,thatforall(x;y[ٲ)2 UUonehasv8jKUNy=n껲(x;y[ٲ)8Ny=n(x;0)je<$p1wfe#b (֍Fnq~Fn+1-q;qˍ6लwhere|(Fnq~)n0nistheFibGonacci-sequence1;1;2;3;5;8;s.WHence|forallirrational6वxUUandally"2[0;1] ;tlimLn!1ڈjNy=n껲(x;y[ٲ)8Ny=n(x;0)jt=q0;&ԍ6लand8theconvergence8isuniform.Butthenforally"2[0;1]thesequence(Ny=^n껲(x;y[ٲ))n06लhassthesamedistributionasNy=^n껲(x;0))n0.&Thus,alsothesequencesT^n)(x;0;g[ٲ))n06लandUUT^n)(x;y[;g))n0GhaveUUthesamedistributionforally"2[0;1]andallg2G(m).BNowletE<[0;1)_ظnQ.bGethesetofthoseirrationalx2[0;1),~iforwhich6(Ny=^n껲(x;0))n0isnotdistributedover accordingto~feg x,2Ythen}feTE :=õEո7H[0;1]6is thesetofpGoints(x;y[ٲ)2  forwhich(Ny=^n껲(x;y[ٲ))n0isnotdistributedover 6accordingto~feg H.NowifEOhad,1assubsetof[0;1),pGositiveLebesguemeasure,1so6wouldUU}feTEasUUasubsetof .qHowever,thisisimpGossiblesince( ;~feg.;Ny=)isergodic.BThusAweseethatE1βisanull-set,andthereforewehaveforalmostallxandforall6वg"2G(m)lthatthesequence(T^n)(x;0;g[ٲ))n0U^isldistributedaccordingto~feg  hm. ÍBलF*rom@Theoremhtml:3 html:,E(html:6 html:)@andLemmahtml:1 html:wehavethefollowingresults,E(html:3 html:)bGeingone6ofUUthem.JM|46html: html:74rKarma$DajaniandCorKri>aaikampN6Corollary8html: html:1.x$L}'et aandmbetwointegers,Rm=2,and let(c1|s;c2)=2[0;1]^2|s.\F;ur- 6thermor}'e,letA(x;c1|s;c2;N)b}'ethecardinalityofthesetA(x;c1|s;c2;N),givenby32Efn;UP1nN;n1u 0;UP">0and+8<1; UO0;Potherwiseyh:̮6Remarks6टhtml: html: 1.T*akingUUc1Ȳequalto1inCorollaryhtml:1 html:atonceyields(html:3 html:).html: html: 2.Usingusomeofthelemmasin[html:JL c html: c]onthenumbGeruofelementsofcertainsets,obtainedmfromG(m)byimpGosingextraconditionsonG(m),atonceyieldseveralpothercorollaries,de.g.,frompTheoremhtml:3 html:,d(html:6 html:),Lemmaphtml:1 html:andLemma(3.10)UUfrom[html:JL c html: c]onehasthefollowingcorollary*.i1Corollary8html: html:2.ACzL}'et aandmbetwointegers,Rm=2,and let(c1|s;c2)=2[0;1]^2|s.\F;ur-thermor}'e,letBq(x;c1|s;c2;N)b}'ethecardinalityofthesetBM۲(x;c1|s;c2;N),givenby<(3fn;UP1nN;n1u E: html::]showed,UUthatforeachz70andforalmostallxonehas獍0&lims3,X1N,!1<$O01F=wfeR (֍logTN_& #f(q[;p);UPqjqx8pjBJW html:],UItoandNakqadashowedUin[html:IN s html: s],thatfor0Ǖz8, ȱ1ȟ&fes2this_resultisaneasyconsequenceofthefactthat( ;~feg.;Ny=)isanergoGdicsystem,andLfromclassicaltheoremsbyLegendreandLGevy*.ZTheirmethoGdcanbeextendedto8 0z71,=butthen( ;~feg.;Ny=)shouldbGereplacedbya`suitable'ergodicsystem,seeealso[html:Ir html:r].yThereasonforthisis,hthatthereexistrationalnumbGersep=q >with(q[;p)6=19and l1l&fes2 1 html:2.L{L}'etva;UPbandmbethreeintegers,suchthatm2vand(a;b;m)=1.Thenforalmostallxand0z7 K1K&fes2 onehasthatthelimit$nR2lims3=N,!1򙖍+#f(q[;p);qjqx8pjoximation$byContinuedFractionsGhtml: html:75NBTheHabGoveresultsalldealwiththepGointwiseconvergenceofergoGdicaverages. 6Classically#Vsuchresultsforcontinuedfractionsarelikeonefaceofacoin,-Vtheother6faceIxbGeingweakconvergenceofprobabilitymeasureswithagivenspGeedofconver-6gence.T*omeconcludethissection,siwewillshowthatalsointhepresentsettingsuch6resultsUUareeasilyobtained.BLeteKZM1 bGeasimplyconnectedsubsetof ,isuchthat@8K=Mٵ`1 ][:::{[`k됲,6whereUUk2Njandeach`iiseitheraverticallinesegment'`i =qf(AiTL;[ٲ);UPCid"Dig6ल(whereUUAid2[0;1]and0CidDKj html:j]thatp(Enq~(K))q=~feg xF(K)8+OG(g[ٟnW);Ǎ6लwhere1theconstantinthebigOG-symbol1isuniformandwhereg"= K3p 3W sM51Ksfeʰg2F=0:616लisUUtheso-calledgoldenme}'an.BNowUUletLG(m)UUbGesomesubsetofG(m),andput*t~3m}feTE(n?(K8L)q:=f(x;g[ٲ)2;UPTn)(x;0;g[ٲ)2K8Lg:6ल(html: html:8)nÍ6In casethecontinued fractionexpansionofxisgivenby(html:4 html:)andthematrixMnq~(x)6isUUgiven|asbGefore|byF.Mnq~(x)q=^d40#WR1415B10p^;4:::J+^dV0iC1V1eBnw^;F6लoneUUclearlyhasgBN}feTEo7nt (K8L)q=f(x;g[ٲ)2;UPNy=n껲(x;0)2K qand~;g[Mnq~(x)2Lg:6लConsequentlyUUonehas,thatcgD(8 hm)(}feTETn fҲ(KL))=cZUR yEO \cmmi5nl(K})" cZ(JE yL]Mk, 0ncmsy51Xn (x)Otdhmd(x)x򍍍=qcZ yEnl(K})$fhm(LM1፴n ӏ(x))d(x)=cZUR yEnl(K})" hm(L)d(x) U=q(Enq~(K))hm(L)=<$ jLjKwfe۟ (֍jG(m)j#'Y~feg)-߲(K)8+OG(g[ٟnW):"6लThusUUwe ndthefollowingtheorem.\6TheoremThtml: html:4.uL}'etS˵K~4 andLG(m)b}'easbefore.7F;urthermore,`let}feTE In(KdL)6b}'ede nedasin(html:8 html:).Then(8 hm)(}feTETn fҲ(KL))q=<$sjLjwfe۟ (֍jG(m)j$~feg*؇(K)+OG(g[ٟnW);6wher}'etheconstantinthebigOG-symbolisuniform.\BलSeveralUUcorollariescanbGeobtainedeasily*.qWeUUmentionhereonlyone.Lz|46html: html:76rKarma$DajaniandCorKri>aaikampN6Corollary8html: html:3.x$L}'eta;UPbandmbethreeintegers,suchthat(a;b;m)=1andm2. 6F;urthermor}'e,letfor0zid1(wherei=1;2)thesetKnq~(z1|s;z2;m)b}'ede nedby唍@f(x;g[ٲ)2;UPn1(x)z1|s;nq~(x)z2Zand|ipn8a(moGdm);qn8b(moGdm)g:6Then>k(Knq~(z1|s;z2;m))q=<$G(z1;z2)wfe&[ (֍J9(m)/l+8OG(g[ٟnW);ˍ6wher}'etheconstantinthebigOG-symbolisuniform.6ट Uhtml: html: 93.1g cmmi12S-expansions InvA[html:Ba X html: X],~}D.BarbGolosishowedvAthatthemethodofJagerandLiardet[html:JL c html: c]canbeextendedtothec}'ontinued0fractionwithoddpartialquotients(OddCF).Essentialin [html:Ba X html: X]is,95thatincasemisevenoneshouldreplaceG(m)byG(m)^09,95whichisasubgroupUUofG(m)ofindex2,generatedbyallmatricesofthetypGeBi^ݟ[ fe0v[ fe1"~feƟg"ޟ~feI0gaA^ҵ;(html: html:9)wheref"2f1;+1gandaisanoGddinteger.UW*edenotethesetofallsuchmatricesfrom^(html:9 html:)byH^07.[IncasemisoGddonehasthatG(m)=G(m)^09.HeuristicallyBarbGolosi's^resultcanbeunderstoodasfollows.In[html:JL c html: c]thefollowinglemmawasobtainedUUfortheRCF.eLemmaThtml: html:2.6L}'eteXUHA:=q^ ^dN50(1N51'N6 2S^;o;UP В2Z 2=mZ^{4;UPm2:圍Thenthegr}'oupG(m)isgeneratedbyH. In^casemisevenH^0%doGesnotcontainthosematricesinHW\forwhichaiseven;theUUremainingmatricesgeneratethesubgroupG(m)^0#ofG(m)insteadofG(m). ApartfromtheOddCFthereareseveralother|classicalandnew|contin-uedfractionalgorithms.VYT*omentionafew: (thene}'arestintegercontinuedfraction(NICF),Hurwitz'singular4c}'ontinuedfraction(SCF),Mikowski'sdiagonalc}'ontin-ue}'dK html:]). Each^5S-expansionisanexampleofa6p0J cmsl10semi-regularcontinuedfraction(SRCF)expansion.qInUUgeneralaSRCFisa niteorin nitefractiono$b0_+<$5"1wfei ECb1S+(#"2lfeR;Agb2+.. 2.+.H"nfe!Agbn^+.. 2.tJ=q[b0|s;UP"1b1;"2b2;UG;"nq~bn;];(html: html:10)49withUU"n8=1;UPb0C2Z 2;bn82N ,UUforn1,UUsub8jecttothecondition唍|"n+1a+8bn81;forN8n1;andUUwiththerestrictionthatinthein nitecasepJ"n+1a+8bn82;in nitelyUUoften.MF|4]Appri>oximation$byContinuedFractionsGhtml: html:77N6MoreoverUUwedemandthat"n^+8bn81forn1. BT*aking n nitetruncationsin(html:10  html:)yieldsa niteorin nitesequenceofrational6numbGers^`rnq~=sn;UPnӸ1,theconvergentsof(html:10  html:).AnSRCF-expansion(html:10  html:)isan6SRCF-expansionUUofxiflim8n!1(Fֵrnq~=sn >=qx.BLetxbGeanirrationalnumber,Handlet(html:10  html:)besomeSRCF-expansionofx.6SuppGoseUthatwehaveforacertainkj0:bk+B+1=1;"k+B+2=1:UTheopGerationby6whichUUthecontinuedfraction(html:10  html:)isreplacedbyhtml:^3|s html:UJѲ[b0|s;UP"1b1;:::UG;"k+B1(bk+B1;"k됲(bk$p+8"k+B+1 );"k+B+1(bk+B+2Ds+81);"k+B+3bk+B+3;:::];6लwhichagainisaSRCF-expansionofx,withconvergents,say*,(cnq~=dn)n1;iscalled 6the^singularizationofthep}'artialquotientFbk+B+1equalto1.dAsFincaseoftheRCF,6setting&񍍟x䍒q'~͖A0;ʲ:=q^d415b040!1-R^6n;x䍑 N1~An ߲:=q^d405"n41õbn.y^7;UPn1;6लandx䍑~UUMn":=qA0|sA1'|jAnq~;UPn0;UUyieldsthatZ#x䍒A~ꕵMnb=q^d乵rn1/Brn4sn1/,&sn>M^GT;UPn0:]6लSimilarlyUUonehasforthenewsequenceofconvergentsUU(cnq~=dn)n14x䍒 ϲ^f#Mn=q^d>cn10 cn4dn1/dn?V^H];UPn0:6लInUU[html:K html:],Section2,itwasshownthat8x䍑ed^b@Mnt,ݲ=x䍑~MnAzfor$n=1;:::;kw81;x䍑^Mn=x䍑~Mn+1a}for.n=k+81;:::U6लand5*x䍒^]Mknٲ=x䍑,l~qMk+B+1ۊ^d+7"k+B+1Pu0/"k+B+1Pu1Zv^ef::č6लF*rompthisitfollowsthat(cnq~=dn)n1cispobtainedfrom(rnq~=sn)n1bypskipping6theUUtermrk됵=sk:UUSeealso[html:K html:],Sections2and4.BARsimplezwaytoderiveastrategyforsingularizationisgivenbyasingularization6ar}'eaS.6De nitionThtml: html:3.zAsubsetS'tfr}'om iscalledasingularisationareaifitsatis esE(i)RlOSZ2Bjand(@8S)=0;(ii)hS([<$33133wfe (֍2fg;1)8nQ k)[0;1];(iii)ㅸNy=(S)\SM=q;:Y㍍6De nitionThtml: html:4.zL}'et(Sbeasingularisationareaandletxbearealirrationalnumber.6TheS-exp}'ansionofxisthatsemi-regularcontinuedfractionexpansionconverging6tox,whichisobtaine}'dfromtheRCF-expansionofxbysingularizingBn+1Aifand6onlyifNy=^n껲(x;0)2ST;UPn0:BलF*romXthesetwoXde nitionsawholetheoryofS-expansionscanbGedeveloped.6See4[html:K html:],+wherealsoseveralexamples(astheaforementionedNICF,SCF#etc.)1care6ट_ff< -:3*html: html:In$casek!=0thiscomesdoÎwntoreplacing(html:10 html:)bytheSRCF$[jbq0+:"i?k+1 /;X"i?k+1(bq2+ >1);X"q3*bq3;"q4bq4;:j::*]:NǠ|46html: html:78rKarma$DajaniandCorKri>aaikampN6लdiscussed.]Essential%inthetheoryofS-expansionsis,%/thatifwedenote nSby%, 6andUUifwede nethemapI.:!UUbyUxI(x;y[ٲ)q:=􍓫8 < :dUONy=(x;y[ٲ);DyNy=(x;y[ٲ)62ST;UONy=^2(x;y[ٲ);DyNy=(x;y[ٲ)2ST;6लthat|sinceI{isaninducedtransformationwithreturntimebGoundedby2|the6system p Խ(;ɵ;I)6isUUergoGdic.qHere gistheprobabilitymeasureonwithdensityʧ<$1]wfe+b (֍~feg()log?2<$1bcwfe(v (֍(18+xy[ٲ)r2µ;(x;y[ٲ)2: 6लF*romբthis,-usingelementarypropGertiesoftheNakqada-Ito-Tanakaբnaturalextension6of theRCF,one ndsthetwo-dimensionalergoGdicsystemunderlyingeveryS-6expansion.qT*oUUbGemoreprecise,letM3:q!R gp28ݲbeUUde nedbybM(x;y[ٲ)q:=􍓫8 < :dUO(x=(18+x);1y[ٲ);sz(x;y[ٲ)2^ :=Ny=(S);UO(x;y[ٲ);sz(x;y[ٲ)2^+ j:=8n^;I6लlet,, S ײ:=еM()andlettheopGeratorS /: S!-( S 3bGe,,de nedbyS(t;v[ٲ):=6वMIM^1 ӏ(t;v[ٲ)UUfor(t;v[ٲ)2 S.qThenUUonehasthefollowingtheorem.O6TheoremThtml: html:5.u([html:K html:])L}'etbetheprobabilitymeasureon S ^withdensityX<$t1wfeB (֍(18~feg?f(S))log?2<$1wfe&M (֍(18+tv[ٲ)r2u;(t;v[ٲ)2 S:F6Theng( S;;S)gformsaner}'godicgsystem.F;urthermore,oifbq: S!N.isgivenby6html: html:!#M[fb(t;v[ٲ)q=8 >>< >>:dUOBq(t);m2ifv該sgnh(t)=1;UPNy=(t;v[ٲ)62ST;UOBq(t)8+1;m2ifv該sgnh(t)=1;UPNy=(t;v[ٲ)2ST;UOBq(t=(18+t))+1;m2ifv該sgnh(t)=1;UPNy=(M^1 ӏ(t;v[ٲ))62ST;UOBq(t=(18+t))+2;m2ifv該sgnh(t)=1;UPNy=(M^1 ӏ(t;v[ٲ))2ST;6ल(html: html:11)#6theniwS(t;v[ٲ)q=^ 4  4 4 4 <$V1Vwfe (֍ȵt    %8b(t;v[ٲ);<$'1wfeKƟ (֍b(t;v)8+sgn(t)vQ|^<;(t;v[ٲ)2 S:WBलInUUviewof(html:10  html:)andTheoremhtml:5 html:onehasthefollowingresult.6TheoremThtml: html:6.uL}'etsZSZ beasingularizationarea,yletm2sZbeaninteger,yandlet6ल S andѭSb}'ede nedasbefore.RF;urthermore,letTS : S0eG(m)6! SeG(m)6b}'ede nedby ETS(t;v[;g)q:=^ 4  4 4 4 <$V1Vwfe (֍ȵt    %8b;<$1wfe! (֍b+"v'V;UPg^db0b"b1b' ^.i0^';E6wher}'eb=b(t;v[ٲ)isde ne}'dasin(html:11  html:)and"=sgnG(t).Then=(H( S8G(m);UPS hm;UPTS)6formsaner}'godicsystem.O|4]Appri>oximation$byContinuedFractionsGhtml: html:79N6ProQof.[De neUUthemapx䍑3~I K^:8G(m)!G(m)UUbyx䍒y~.I(x;y[;g)q:=􍓫8 < :dUOT(x;y;g);L[sNy=(x;y)62ST; UOT^2(x;y;g);L[sNy=(x;y)2ST:x6लThenZR(<3G(m);~feg a hm;x䍑=~Iٙ)formsanergoGdicsystem,[whichis|inviewof(html:10  html:), 6TheoremShtml:5 html:andthefactthatincaseoftheRCFSalwaysSŵ"n ल=o&+1|metrically6isomorphic to( SG|xG(m);UPS hm;UPTS) viathemapM:G(m)!G(m),6givenUUby"oU M(x;y[;g)q:=8 >>< >>:\UO(x;y;g);Ҳ(x;y)2^+;UO(x=(18+x);1y;g^db1%*0F1%*1/*^6);Ҳ(x;y)2^:"UCnBलInUUcaseSZ isasingularizationarea,onehas(seealso[html:K html:],Section4)t0~feg͞(S)18<$llogV(g+1)lwfe+ (֍ logPp22q=0:30575UG:ch6लConversely*,%forQ/everysj׸2(0;UP0:30575]thereexistin nitelymanysingulariza-6tion'areasS8$ suchthat~feg -(S)=s.F*oreveryirrationalnumbGerxandevery6singularization3areaSUde nethemonotonicallyincreasingarithmeticalfunction6वnS(kP)=nS(kP;x)UUbyrk됵=sk=p:nmS(k+B)%=q:nmS(k+B).qThenUUforalmostallx箍姲lim´k+B!1<$nS(kP)wfe (֍ `k=<$1wfe%ן (֍18~feg?f(S)/M;񡍑6लseeUUalso[html:K html:],Theorem(4.13).BInspiteofthis,ԢitfollowsfromTheoremhtml:6 html:thatforanyS-expansionandfor6almost~everyxthesequenceofnumerators(rnq~)n1qanddenominators(sn)n16लofjtheS-convergentsj(rnq~=sn)n1Xofxhave|moGdm|thesameasymptotic6bGehaviourasthesequenceofnumerators(pnq~)n1anddenominators(qn)n1of6theUURCF-convergentsofx.BT*oUUbGemoreprecise,wehavethefollowingcorollary*._6Corollary8html: html:4.x$L}'et r;UPsandmbethreeintegers,suchthatm⇸2 and(r;s;m)=1.6ThenforalmostallxonehasfXdlims3TjoN,!1<$p_IJ1nOşwfe  (֍Nx#^ *n;UP1nN;^dIrnĵsnB^*vq^d4pq4^%(moGdm)^Ԓ?=<$W1Owfe (֍J9(m)!>:.6Ac9knowledgementsòW*ewanttothankthereferee,^HenkJagerandMarkStein-6bGergerUUforseveralremarkswhichimprovedthepresentationofthispapGer.6ट html: html: 9References[html: html:Ba] D.B}Barb html:Br] J.R.BroÎwn,,Ergodic?TheoryandTWwopologicalDynamics,,AcademicPress,NewYJork,San FJrancisco,XLondon,1976.[html: html:BJW]W.)Bosma,?H.Jager,andF.Wiedijk,Someametricalobservationsontheapproximation by~continuedfractions,XIndag.Math.,45(1983),281{299.Ph|46html: html:80rKarma$DajaniandCorKri>aaikampN6ह[html: html:DK]OjK.DaxjaniandC.KraaikampGeneralization"ofatheoremofKusmin,Monatsh.Math., Oj118X(1994),55{73.6[html: html:DP]OjR.DescomÎb html:E]OjPJ.XErdos,Some~resultsonDiophantineapproximation,XActaArith.,V(1959),359{369.6[html: html:H]OjS..Hartman,SurScuneconditionsupplԟementairedanslesapproximationsdiophantiques,OjColl.XMath.,2(1949),48{51.6[html: html:IN]OjSh.ItoandH.Nakada,.JOnPjnaturalextensionsoftransformationsrelatedtoDiophantineap-Ojproximations,Pro html:I]OjSh.NcIto,lAlgorithmswithmediantconvergentsandtheirmetricaltheory,lOsakaNcJ.Math.,Oj26X(1989),557{578.6[html: html:JL]OjH.JagerandPJ.Liardet,Distributions~arithmԟetiquesdesdemoninateursdeconvergents~deOjfractions~continues,XIndag.Math.,50(1988),181{197.6[html: html:Kok]OjJ.7Koksma, qSur@Tl'approximationdesnombresirrationelssousuneconditionsupplԟementaire,OjSimonXStevin28(1951),199{202.6[html: html:K]OjC.Kraaikamp,'A"new"classofcontinued"fractionexpansions,'ActaArithm.,L>VIEIƹ(1991),Oj1{39.6[html: html:L]OjPJ.h^Liardet, Distributions#ArithmԟetiquesdesDemoninateursdeConvergentsdeFWwractionsOjContinues,1Sem.Th.desNomÎbresBordeaux1986{1987,Exp html:M]OjR.Mo html:NIT]OjH.ztNakada,Sh.Ito,andS.TJanaka,S.OntheinvariantmeasureforthetransformationOjassociated~withsomereal~continuedfraction,XKeioEng.Rep.,30(1977),159{175.6[html: html:Na]OjH.+Nakada,) Metrical theoryforaclassofcontinuedfractiontransformationsandtheirOjnatural~extensions,XTJokyÎoJ.Math.,4(1981),399{426.6[html: html:No]OjV.)N.Nolte,SomeYprobabilisticresultsontheconvergentsofcontinuedfractions,OjIndag.XMath.(N.S.)1(1990),381{389.6[html: html:R]OjV.wA.Rohlin,ExacteendomorphismsofaLebesgueespace,Izv.wAkad.NaikSSSR,Ser.Mat.,Oj24X(1960);EnglishAMStranslation,Series2,39(1969),1{36.6[html: html:R-N]OjC.PRyll-Nardzewski,OOn,}theergodic,}theorems(I html:Universiteit#Utrecht,VDepuUartmentofMathematics,VBudapestlaan6,P.O.Box80.000,63508#T@AUtrecht,theNetherlandsB)html:daxjani@math.ruu.nl html:Bhtml: html:Thomas[5StieluUtjesInstituteforMathematicsandTechnischeUniversiteitDelft,6FaculuUtyqITS,DepartmentofMathematics(SSOR),MekelGweg4,2628CDoDelft,the6NetherlandsB1html:c.kraaikamp@tÎwi.tudelft.nl html:BThisXpaphttp://nyjm.albanyJ.edu:8000/j/1998/3A-5.html html:.html: html:p;|4 6p0J cmsl101g cmmi12.F C cmbxti10-2@cmbx8,K`yff cmdunh10*"V cmbx10)': cmti10% msam10#- cmcsc10"N cmbx12!XQ cmr12 Nff cmbx12#fcmti8fcmcsc8q% cmsy6K cmsy8;cmmi62cmmi8Aacmr6|{Ycmr8 !", cmsy10 O!cmsy7 0ncmsy5 b> cmmi10 0ercmmi7O \cmmi5K`y cmr10ٓRcmr7Zcmr5u cmex10&