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The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto April 3, 2010 The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 1 / 51 1 D-continued fraction or the ﬂipped CF 2 Examples 3 From regular CF to ﬂipped CF 4 Periodic expansions and quadratic irrationals 5 Ergodic Properties 6 Simulations of invariant density The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 2 / 51 The classical case It is well-known that every real number x can be written as a ﬁnite (in case x ∈ Q) or inﬁnite (regular) continued fraction expansion (RCF) of the form 1 x = a0 + , 1 a1 + 1 a2 + · · · + an + · · · where a0 ∈ Z is such that x − a0 ∈ [0, 1), i.e. a0 = x , and an ∈ N for n ≥ 1. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 3 / 51 The classical case The partial quotients an are given by 1 an = an (x) = , if T n−1 (x) = 0, T n−1 (x) where T : [0, 1) → [0, 1) is the continued fraction (or: Gauss) map, deﬁned by 1 1 T (x) = − , if x = 0, x x and T (0) = 0. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 4 / 51 The classical case 1 1 . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . .. . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . .. . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . .. .. . . . . . . . . .. . . . . . . . .. . . .. ••• . . . . . . . . . . . . . . . . . . . . . . . . .. .. . .. .. . . . . . . . . .. .. . . . . .. . . . .. . . . . . . . . . . . .. . . . . . . . . . .. .. . . . . . . . .. . .. . . . . . . .. . . . .. . . . . . . . . . .. .. . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . .. .. . . . . . . . . .. . . . . . . . . .. . . . . . . .. . .. .. . . . . . . . . . . .. . . . . . . .. .. . . . . . . . . . .. .. . . . . . . . . . .. . . . . . . . . . . .. . . . . .. .. . .. .. . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . .. .. . . . . . .. .. .. . . . . . . . . . .. . . . . . . . . .. .. . . . . . . . . . .. .. .. . . . . . . . . . . . . . . . . . . . .. ... ... .. . . . . . . ... ... . . . . . . . . . . ... ... . . . . . . . . .. ... ... . . . . . . . . . . ... . . . . . . . .. ... . . . . .. . . .. . 11 1 1 1 0 65 4 3 2 1 The continued fraction map T . The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 5 / 51 D-continued fraction or the Flipped CF Let D ⊂ [0, 1] be a Borel measurable subset of the unit interval, then we deﬁne the map TD : [0, 1) → [0, 1) by 1 1 + 1 − , if x ∈ D x x TD (x) := 1 − 1 , if x ∈ [0, 1) \ D. x x The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 6 / 51 D-continued fraction 1 .. .. ... .. .. .... .... .. .. ..... ... ... ... ... ... .... ... .. .... . ... ....... .... 1 .. ... ... 2 .. ... .... . 1 y = x − n = T (x) . . . y..= n + 1 − x = 1 − T (x) 1 .. ..... . .. ... . .... .. .... .. .... .. . ..... ... . . 1 2 1 0 n+1 2n+1 n The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 7 / 51 D-continued fraction −1, if x ∈ D Setting ε1 = ε1 (x) = and +1, if x ∈ [0, 1) \ D, 1/x + 1, if x ∈ D d1 = d1 (x) = 1/x , if x ∈ [0, 1) \ D, it follows from deﬁnition of TD that 1 TD (x) = ε1 − d1 . x The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 8 / 51 D-continued fraction n−1 Setting for n ≥ 1 for which TD (x) = 0, n−1 n−1 dn = d1 (TD (x)), εn = ε1 (TD (x)), we ﬁnd that 1 1 x= = ε1 . d1 + ε1 TD (x) d1 + εn−1 d2 + · · · + n dn + εn TD (x) The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 9 / 51 D-continued fraction For each n ≥ 1, 1 x= ε1 . d1 + εn−1 d2 + · · · + n dn + εn TD (x) One needs to show that as n → ∞, the above converges to 1 x= ε1 . d1 + εn−1 d2 + · · · + . dn + . . The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 10 / 51 D-continued fraction For each n ≥ 1, 1 x= ε1 . d1 + εn−1 d2 + · · · + n dn + εn TD (x) One needs to show that as n → ∞, the above converges to 1 x= ε1 . d1 + εn−1 d2 + · · · + . dn + . . The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 10 / 51 D-continued fraction We have 1 x= ε1 . d1 + εn−1 d2 + · · · + n dn + εn TD (x) The nth D-convergent of x is pn 1 = ε1 . qn d1 + εn−1 d2 + · · · + dn The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 11 / 51 D-continued fraction We have 1 x= ε1 . d1 + εn−1 d2 + · · · + n dn + εn TD (x) The nth D-convergent of x is pn 1 = ε1 . qn d1 + εn−1 d2 + · · · + dn The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 11 / 51 D-continued fraction Using similar methods as in the regular CF, one can show that n pn + pn−1 TD (x)εn x= n . qn + qn−1 TD (x)εn gcd(pn , qn ) = 1. n−1 pn−1 qn − pn qn−1 = (−1)n k=1 εk = ±1. pn (−1)n ( n εk )TD (x) k=1 n x− = n . qn qn (qn + qn−1 εn TD (x)) The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 12 / 51 D-continued fraction Using similar methods as in the regular CF, one can show that n pn + pn−1 TD (x)εn x= n . qn + qn−1 TD (x)εn gcd(pn , qn ) = 1. n−1 pn−1 qn − pn qn−1 = (−1)n k=1 εk = ±1. pn (−1)n ( n εk )TD (x) k=1 n x− = n . qn qn (qn + qn−1 εn TD (x)) The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 12 / 51 D-continued fraction Using similar methods as in the regular CF, one can show that n pn + pn−1 TD (x)εn x= n . qn + qn−1 TD (x)εn gcd(pn , qn ) = 1. n−1 pn−1 qn − pn qn−1 = (−1)n k=1 εk = ±1. pn (−1)n ( n εk )TD (x) k=1 n x− = n . qn qn (qn + qn−1 εn TD (x)) The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 12 / 51 D-continued fraction Using similar methods as in the regular CF, one can show that n pn + pn−1 TD (x)εn x= n . qn + qn−1 TD (x)εn gcd(pn , qn ) = 1. n−1 pn−1 qn − pn qn−1 = (−1)n k=1 εk = ±1. pn (−1)n ( n εk )TD (x) k=1 n x− = n . qn qn (qn + qn−1 εn TD (x)) The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 12 / 51 D-continued fraction Using similar methods as in the regular CF, one can show that n pn + pn−1 TD (x)εn x= n . qn + qn−1 TD (x)εn gcd(pn , qn ) = 1. n−1 pn−1 qn − pn qn−1 = (−1)n k=1 εk = ±1. pn (−1)n ( n εk )TD (x) k=1 n x− = n . qn qn (qn + qn−1 εn TD (x)) The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 12 / 51 D-continued fraction From pn (−1)n ( n εk )TD (x) k=1 n x− = n (x)) , qn qn (qn + qn−1 εn TD it follows that n TD (x) pn 1 x− = n < n → 0. qn qn (qn + qn−1 εn TD (x)) |qn (qn + qn−1 εn TD (x))| The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 13 / 51 D-continued fraction From pn (−1)n ( n εk )TD (x) k=1 n x− = n (x)) , qn qn (qn + qn−1 εn TD it follows that n TD (x) pn 1 x− = n < n → 0. qn qn (qn + qn−1 εn TD (x)) |qn (qn + qn−1 εn TD (x))| The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 13 / 51 Example- Folded α-expansion In 1997, Marmi, Moussa and Yoccoz modiﬁed Nakada’s α-expansions to the folded or Japanese continued fractions, with underlying map 1 1 Tα = − , for 0 < x < α, x = 0; Tα (0) = 0, x x α where x α = min{p ∈ Z : x < α + p}. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 14 / 51 Example- Folded α-expansion Folded α-expansions can also be described as D-expansions with ∞ 1 1 D= , ; n+1 n+α n=1 The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 15 / 51 Example- Folded α-expansion 1 α . . . . . . ... . . . .. ... . . . . . ... . . .. . ... . . . . . . ... . . . . . ... .. . .. ... . . . . . . . . .. .. ... . . . . ... . . . ... ... . . .. ... . . . .. .. ... . . . .. ... .. .. ... . . . .. . .. ... 1−α . . . . ... . . . . . . . .. .. . ... . . . . . . .. .. .. . ... . . .. . .. .. . ... .... . . . . . . .. .. .. . .... . . . . . . . . . .. .. .. . .. .... .. .. .. . .. .. . .... .. .. .. .... .. .. .. .. .. .. .... .. .. ..... .. . .. .... .. . ..... . .. ... 0 1 3+α 1 3 1 2+α 1 2 1 1+α 1 The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 16 / 51 Example- Backward CF Let D = [0, 1), and let x ∈ [0, 1). In this case [0, 1) \ D = ∅, so we always use the map 1 1 TD = 1 + − , x x and we will get an expansion for x of the form 1 x= = [0; 1/d1 , −1/d2 , . . . ]; −1 d1 + d2 + . . . The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 17 / 51 Example- Backward CF It is a classical result that every x ∈ [0, 1) \ Q has a unique backward continued fraction expansion of the form 1 x =1− = [0; −1/c1 , −1/c2 , . . . ], 1 c1 − c2 − . . . where the ci s are all integers greater than 1. This continued fraction is generated by the map 1 1 Tb (x) = − , 1−x 1−x that we obtain from TD via the isomorphism ψ : x → 1 − x, i.e., ψ ◦ Tb = TD ◦ ψ The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 18 / 51 Example- Backward CF 1 . .. .. 1 .. . . . . . .. .... .. . . . . .. .. .... .. . . . . . . . .... . . . . . . . . .. .... . .. . . . . . . . .. ... .. . . . . . . . ... . . . . . . . . . ... .. . . . . . . .. ... .. . . . . . . . . .. .. . . . . . . . . .. .. . . . . . . . . . . .. .. . . .. .. . . .. .. . . . . . . .. .. . . . . . . . . .. . .. .. . . . . ......... .. ........ .. . . . . .. .. . . .. .. . . . . .. . .. y. = 3 −.. 1 .. ... . . . . . . . . . .. x ... . . . . ... y = 1−x − 1 .. 1 . . . . . . .. . . . .. .. ... .. . . . . . .. ........ . . ... . ...... . . . . . . .. ... ... . .. ... . 1 . . . . . .. y = 2 − x ... .. y =.1−x − 2 . .... . 1 . .. . .. .. .. . . . . .... . . . . . . . .. ..... . . . . 1 1 1 1 2 3 0 4 3 2 1 0 2 3 4 1 The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 19 / 51 Example- Odd and Even CF Setting 1 1 D := Dodd = , , n even n+1 n one easily ﬁnds that the D-expansion for every x ∈ [0, 1) only has odd c partial quotients dn . In case D := Deven = Dodd , the partial quotients are always even. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 20 / 51 Example- CF without a particular digit Fix a positive integer , and suppose that we want an expansion in which the digit never appears, that is an = for all n ≥ 1. Now just take 1 D = ( l+1 , 1 ] in order to get an expansion with no digits equal to . l The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 21 / 51 Example- CF without a particular digit An expansion with no digit equals 3. 1 . . .... . . . .... . . . . .. ... .. .. .. . . . .. .. .. .. . . . . . .. ... .. ... . . .. . . . . .. ... ..... . . . . .. .. ......... ... .... .. . . .. .. ... ... . . . . . .. .. . ... ... y = x −1 1 . . . . .. .. ... . . . . ... .. ... . ... . . . . .. .. ... . . .. . ... . . . . . ... . . . .. ... . .. . ... . . .. . . .. . . .. . ... ... . .. . . ..... . ... ... . .. ... . . .. . . . . ... . . . . ... . . . . . . y =3− x 1 ... . . . ... . . . . ... . . . . ... . . . . . .... . . . . . . .... .. .. .. .... . .. .... .. .. .. .... . .. .... .. .. ..... . .. ..... . . 1 1 1 0 4 3 2 1 The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 22 / 51 From regular CF to ﬂipped CF- Singularizations Let a, b be positive integers, ε = ±1, and let ξ ∈ [0, 1). A singularization is based on the identity ε −ε a+ =a+ε+ . 1 b+1+ξ 1+ b+ξ The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 23 / 51 Singularizations To see the eﬀect of a singularization on a continued fraction expansion, let x ∈ [0, 1), with continued fraction expansion x = [a0 ; ε0 /a1 , ε1 /a2 , ε2 /a3 , . . . ]. and suppose that for some n ≥ 0 one has an+1 = 1; εn+1 = +1, an + εn = 0 Singularization then changes the above continued fraction expansion into [a0 ; ε0 /a1 , . . . , εn−1 /(an + εn ), −εn /(an+2 + 1), . . . ]. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 24 / 51 Insertions An insertion is based upon the identity 1 −1 a+ =a+1+ , b+ξ 1 1+ b−1+ξ where ξ ∈ [0, 1) and a, b are positive integers with b ≥ 2. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 25 / 51 Insertions let x ∈ [0, 1), with continued fraction expansion x = [a0 ; ε0 /a1 , ε1 /a2 , ε2 /a3 , . . . ]. Suppose that for some n ≥ 0 one has an+1 > 1; εn = 1. An insertion ‘between’ an and an+1 will change the above CF into [a0 ; ε0 /a1 , . . . , εn−1 /(an + 1), −1/1, 1/(an+1 − 1), . . . ]. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 26 / 51 Insertions/Singularizations Every time we insert between an and an+1 we decrease an+1 by 1, i.e. the new (n + 2)th digit equals an+1 − 1. This implies that for every n we can insert between an and an+1 at most (an+1 − 1) times. On the other hand, suppose that an+1 = 1 and that we singularize it. Then both an and an+2 will be increased by 1, so we can singularize at most one out of two consecutive digits The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 27 / 51 From Regular CF to Flipped CF 1 1 For n ∈ N, let x ∈ In := n+1 , n , so that the RCF-expansion of x looks like 1 x= , 1 n+ .. . and suppose that x ∈ In ∩ D = ∅. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 28 / 51 From regular to ﬂipped 1 .. .. ... .. .. .... .... .. .. ..... ... ... ... ... ... .... ... .. .... . ... ....... .... 1 .. ... ... 2 .. ... .... . 1 y = x − n = T (x) . . . y..= n + 1 − x = 1 − T (x) 1 .. ..... . .. ... . .... .. .... .. .... .. . ..... ... . . 1 2 1 0 n+1 2n+1 n The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 29 / 51 From Regular CF to Flipped CF via insertions 1 2 Suppose x ∈ n+1 , 2n+1 ∩ D, then the regular CF is 1 x= = [0; n, 1, a3 , . . . ]. 1 n+ 1 1+ a3 + ξ Singularizing the second digit, equal to 1, in the previous expansion we ﬁnd 1 x= = [ 0; 1/(n + 1), −1/(a3 + 1), . . . ]. −1 n+1+ a3 + 1 + ξ The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 30 / 51 From Regular CF to Flipped CF via insertions 1 2 Suppose x ∈ n+1 , 2n+1 ∩ D, then the regular CF is 1 x= = [0; n, 1, a3 , . . . ]. 1 n+ 1 1+ a3 + ξ Singularizing the second digit, equal to 1, in the previous expansion we ﬁnd 1 x= = [ 0; 1/(n + 1), −1/(a3 + 1), . . . ]. −1 n+1+ a3 + 1 + ξ The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 30 / 51 From Regular CF to Flipped CF via insertions We now look at the D-CF of x. We have 1 1 1 TD (x) = n + 1 − = 1 − T (x) = 1 − = , x 1 a3 + 1 + ξ 1+ a3 + ξ Thus the D-expansion of x is 1 x= −1 n+1+ a3 + 1 + ξ 1 2 TD acts as a singularization on n+1 , 2n+1 ∩ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 31 / 51 From Regular CF to Flipped CF via insertions We now look at the D-CF of x. We have 1 1 1 TD (x) = n + 1 − = 1 − T (x) = 1 − = , x 1 a3 + 1 + ξ 1+ a3 + ξ Thus the D-expansion of x is 1 x= −1 n+1+ a3 + 1 + ξ 1 2 TD acts as a singularization on n+1 , 2n+1 ∩ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 31 / 51 From Regular CF to Flipped CF via insertions We now look at the D-CF of x. We have 1 1 1 TD (x) = n + 1 − = 1 − T (x) = 1 − = , x 1 a3 + 1 + ξ 1+ a3 + ξ Thus the D-expansion of x is 1 x= −1 n+1+ a3 + 1 + ξ 1 2 TD acts as a singularization on n+1 , 2n+1 ∩ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 31 / 51 From Regular CF to Flipped CF via insertions 2 1 Suppose x ∈ 2n+1 , n ∩ D. RCF-expansion of x is given by 1 x= , 1 n+ a2 + ξ where ξ ∈ [0, 1] and with a2 ≥ 2 because T (x) ≤ 1/2. An insertion after the ﬁrst partial quotient yields 1 x= . −1 n+1+ 1 1+ a2 − 1 + ξ The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 32 / 51 From Regular CF to Flipped CF via insertions 2 1 Suppose x ∈ 2n+1 , n ∩ D. RCF-expansion of x is given by 1 x= , 1 n+ a2 + ξ where ξ ∈ [0, 1] and with a2 ≥ 2 because T (x) ≤ 1/2. An insertion after the ﬁrst partial quotient yields 1 x= . −1 n+1+ 1 1+ a2 − 1 + ξ The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 32 / 51 From Regular CF to Flipped CF via insertions 2 1 Suppose x ∈ 2n+1 , n ∩ D. RCF-expansion of x is given by 1 x= , 1 n+ a2 + ξ where ξ ∈ [0, 1] and with a2 ≥ 2 because T (x) ≤ 1/2. An insertion after the ﬁrst partial quotient yields 1 x= . −1 n+1+ 1 1+ a2 − 1 + ξ The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 32 / 51 From Regular CF to Flipped CF via insertions Since x ∈ (1/(n + 1), 1/n] ∩ D, the D-expansion of x is given by 1 x= . n + 1 + −1(TD (x)) Computing TD (x) we ﬁnd 1 1 TD (x) = 1 − T (x) = 1 − = , a2 + ξ 1 1+ a2 − 1 + ξ so that the D-expansion of x is 1 x= . −1 n+1+ 1 1+ a2 − 1 + ξ 2 1 Thus we see that TD acts as an insertion on 2n+1 , n ∩ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 33 / 51 From Regular CF to Flipped CF via insertions Since x ∈ (1/(n + 1), 1/n] ∩ D, the D-expansion of x is given by 1 x= . n + 1 + −1(TD (x)) Computing TD (x) we ﬁnd 1 1 TD (x) = 1 − T (x) = 1 − = , a2 + ξ 1 1+ a2 − 1 + ξ so that the D-expansion of x is 1 x= . −1 n+1+ 1 1+ a2 − 1 + ξ 2 1 Thus we see that TD acts as an insertion on 2n+1 , n ∩ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 33 / 51 From Regular CF to Flipped CF via insertions Since x ∈ (1/(n + 1), 1/n] ∩ D, the D-expansion of x is given by 1 x= . n + 1 + −1(TD (x)) Computing TD (x) we ﬁnd 1 1 TD (x) = 1 − T (x) = 1 − = , a2 + ξ 1 1+ a2 − 1 + ξ so that the D-expansion of x is 1 x= . −1 n+1+ 1 1+ a2 − 1 + ξ 2 1 Thus we see that TD acts as an insertion on 2n+1 , n ∩ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 33 / 51 From Regular CF to Flipped CF via insertions Since x ∈ (1/(n + 1), 1/n] ∩ D, the D-expansion of x is given by 1 x= . n + 1 + −1(TD (x)) Computing TD (x) we ﬁnd 1 1 TD (x) = 1 − T (x) = 1 − = , a2 + ξ 1 1+ a2 − 1 + ξ so that the D-expansion of x is 1 x= . −1 n+1+ 1 1+ a2 − 1 + ξ 2 1 Thus we see that TD acts as an insertion on 2n+1 , n ∩ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 33 / 51 From regular to ﬂipped 1 .. .. ... .. .. .... .... .. .. ..... ... ... ... ... ... .... ... .. .... . ... ....... .... 1 .. ... ... 2 .. ... .... . 1 y = x − n = T (x) . . . y..= n + 1 − x = 1 − T (x) 1 .. ..... . .. ... . .... .. .... .. .... .. . ..... ... . . 1 2 1 0 n+1 2n+1 n The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 34 / 51 From regular to ﬂipped CF Theorem: Let x be a real irrational number with RCF-expansion 1 x = a0 + , 1 a1 + 1 a2 + · · · + an + · · · and with tails tn = [0; 1/an+1 , 1/an+2 , . . . ]. Let D be a measurable subset of [0, 1). Then the following algorithm yields the D-expansion of x: (1) Let m := inf{m ∈ N ∪ {∞} : tm ∈ D and εm = 1}. In case m = ∞, the RCF-expansion of x is also the D-expansion of x. In case m ∈ N: (i) If am+2 = 1, singularize the digit am+2 in order to get x = [a0 ; . . . , 1/(am+1 + 1), −1/am+3 , . . . ]. (ii) If am+2 = 1, insert −1/1 after am+1 to get x = [a0 ; . . . , 1/am+1 + 1, −1/1, 1/(am+2 − 1), . . . ]. (2) Replace the RCF-expansion of x with the continued fraction obtained in [(1)], and let tn denote the new tails. Repeat the above procedure. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 35 / 51 Quadratic Irrationals A number x is called quadratic irrational if it is a root of a polynomial ax 2 + bx + c with a, b, c ∈ Z, a = 0, and b 2 − 4ac not a perfect square (i.e., if x is an irrational root of a quadratic equation). The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 36 / 51 Quadratic irrationals and regular CF Theorem: A number x is a quadratic irrational number if and only if x has an eventually periodic regular continued fraction expansion. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 37 / 51 Quadratic irrationals and ﬂipped CF we say that a D-expansion of x is purely periodic of period-length m, if the initial block of m partial quotients is repeated throughout the expansion, that is, if akm+1 = a1 , . . . , a(k+1)m = am , and εkm+1 = ε1 , . . . , ε(k+1)m = εm for every k ≥ 1. The notation for such a continued fraction is x = [a0 ; ε0 /a1 , ε1 /a2 , . . . , εm−1 /am , εm ]. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 38 / 51 Quadratic irrationals and ﬂipped CF An (eventually) periodic continued fraction consists of an initial block of length n ≥ 0 followed by a repeating block of length m and it is written as x = [a0 ; ε0 /a1 , ε1 /a2 , . . . , εn−1 /an , εn /an+1 , . . . , εn+m−1 /an+m , εn+m ]. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 39 / 51 Quadratic irrationals and ﬂipped CF Theorem: Let D be a measurable subset in the unit interval. Then a number x is a quadratic irrational number if and only if x has an eventually periodic D-expansion. The proof is based on the result for the regular CF, together with the fact that a D-expansion is obtained from the regular CF by singularizations and insertions. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 40 / 51 Quadratic irrationals and ﬂipped CF Theorem: Let D be a measurable subset in the unit interval. Then a number x is a quadratic irrational number if and only if x has an eventually periodic D-expansion. The proof is based on the result for the regular CF, together with the fact that a D-expansion is obtained from the regular CF by singularizations and insertions. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 40 / 51 Invariant measures Theorem: Suppose D is a countable union of disjoint intervals, then TD admits at most a ﬁnite number of ergodic exact TD -invariant measures absolutely continuous with respect to Lebesgue measure. The proof of this result relies on a Theorem by Rychlik where he characterized ergodic measures of a certain family of piecewise continuous maps. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 41 / 51 Invariant measures Theorem: Suppose D is a countable union of disjoint intervals, then TD admits at most a ﬁnite number of ergodic exact TD -invariant measures absolutely continuous with respect to Lebesgue measure. The proof of this result relies on a Theorem by Rychlik where he characterized ergodic measures of a certain family of piecewise continuous maps. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 41 / 51 Some examples ∞ Let D = ( 2 , 1) ∪ 3 n=2 1 2 n+1 , 2n+1 1 .. .. .... .. .... .. .... .. .. .... .. .... .. ... .. ... .. .... ... ... ... ..... ... ... ..... ... ... ... .. ..... 1 . . . .. ... .. .. 2 . .. .. . . .. . . .. . . . . .. . . . .. .. . . .. .. . .. . .. .. . . .. . . .. . . . . . . .. . . . . . . . .. .. . . . .. .. . . .. .. . . . . . .. .. . . .. . .. . .. . . .. .. . .. . .. .. . ... . ... . ... . .. .. . . .. . .. .. . . . . 1 1 2 1 2 4 3 5 2 3 0 1 The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 42 / 51 Some examples There are two ergodic components absolutely continuous with respect to Lebesgue measure. One is ﬁnite with support [0, 1/2] (this continued fraction is in fact the folded nearest integer continued fraction), and the other is σ-ﬁnite with support (1/2, 1) (this one is in essence Ito’s mediant map) The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 43 / 51 Some examples ∞ 1 1 Suppose D = i=0 ni +1 , ni , where (ni )i≥0 is a sequence of positive integers. [0, 1) is the only TD forward invariant set. Hence, TD admits a unique ergodic invariant measure equivalent to Lebesgue measure on [0, 1). Furthermore, it is ﬁnite if and only if D doesn’t contain 1, and σ-ﬁnite inﬁnite if 1 ∈ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 44 / 51 Some examples ∞ 1 1 Suppose D = i=0 ni +1 , ni , where (ni )i≥0 is a sequence of positive integers. [0, 1) is the only TD forward invariant set. Hence, TD admits a unique ergodic invariant measure equivalent to Lebesgue measure on [0, 1). Furthermore, it is ﬁnite if and only if D doesn’t contain 1, and σ-ﬁnite inﬁnite if 1 ∈ D. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 44 / 51 Some examples ∞ 1 1 Let α ∈ (0, 1), and suppose D = n=1 n+1 , n+α 1 α . . . . . . ... . . . .. ... . . . . . ... . . .. . ... . . . . . . ... . . . . . ... .. . .. ... . . . . . . . . .. .. ... . . . . ... . . . ... ... . . .. ... . . . .. .. ... . . . .. ... .. .. ... . . . .. . . .. ... 1−α . . . . . . . . ... . . .. . ... . . . . . . . .. .. .... ... ... . . . . . . . .. .. .. .. .... . . .. . . . . . . . .. ... .. . .. .... .... . . .. .. .... .. .. .. . .. .. .... .. .. .. . .... .. . .. .. .. .. ..... .. .. . .... .... ..... . . ... 0 1 3+α 1 3 1 2+α 1 2 1 1+α 1 The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 45 / 51 Some examples TD has one ergodic component absolutely continuous with respect to Lebesgue measure, which is ﬁnite and with support the interval [0, max{α, 1 − α}). 1 α . . . . . . ... . . . .. ... . . . . . ... . . .. . ... . . . . . . ... . . . . . ... .. . .. ... . . . . . . . . .. .. ... . . . . ... . . . ... ... . . .. ... . . . .. .. ... . . . .. ... .. .. ... . . . .. . . . .. ... 1−α . . . . . .. ... . . . . .. . ... . . . . . . . . .. .. .. .. .. ... ... . . . . .. .... . . . . . . . .. .. .. .... . . . . . . . .. .. .. .. .... .... .. .. .. .. .. .. .. .... .. .. .. .. . .... .. .. .. .. .. ..... .. . . .. . ... ..... .. . .. ... 0 1 3+α 1 3 1 2+α 1 2 1 1+α 1 The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 46 / 51 Simulations of invariant densities Finding the density of an invariant measure is in general an extremely hard problem. To get an idea of the density, we use Birkhoﬀ’s Ergodic Theorem: for a measurable set A, and for a.e. x n−1 1 µ(A) = lim 1A (T i (x)), n→∞ n i=0 where 1A is the characteristic function of A. We make histograms by counting the number of times that the orbit of a point lies in a particular interval. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 47 / 51 Simulations of invariant densities Finding the density of an invariant measure is in general an extremely hard problem. To get an idea of the density, we use Birkhoﬀ’s Ergodic Theorem: for a measurable set A, and for a.e. x n−1 1 µ(A) = lim 1A (T i (x)), n→∞ n i=0 where 1A is the characteristic function of A. We make histograms by counting the number of times that the orbit of a point lies in a particular interval. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 47 / 51 Simulations of invariant densities Finding the density of an invariant measure is in general an extremely hard problem. To get an idea of the density, we use Birkhoﬀ’s Ergodic Theorem: for a measurable set A, and for a.e. x n−1 1 µ(A) = lim 1A (T i (x)), n→∞ n i=0 where 1A is the characteristic function of A. We make histograms by counting the number of times that the orbit of a point lies in a particular interval. The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 47 / 51 Invariant Density D = [1/4, 1/3) 1.4 1.2 1.0 0.8 Density 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 x The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 48 / 51 Invariant Density D = [1/2, 1) 30 25 20 Density 15 10 5 0 0.0 0.2 0.4 0.6 0.8 1.0 x The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 49 / 51 Invariant Density D = [0.3, 0.45) 1.4 1.2 1.0 0.8 Density 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 x The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 50 / 51 The End The Flipped Continued Fraction Karma Dajani Joint work with C. Kraaikamp and V. Masarotto () April 3, 2010 51 / 51

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Quadratic irrationals, Birkhoﬀ's Ergodic Theorem, Lebesgue measure, Invariant Density, invariant measure, Continued Fraction, Karma Dajani, fraction expansion, n + 1

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posted: | 6/3/2011 |

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