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 A162910 Denominators of Bird tree fractions. 15
 1, 2, 1, 3, 3, 1, 2, 5, 4, 4, 5, 2, 1, 3, 3, 8, 7, 5, 7, 7, 5, 7, 8, 3, 3, 1, 2, 5, 4, 4, 5, 13, 11, 9, 12, 9, 6, 10, 11, 11, 10, 6, 9, 12, 9, 11, 13, 5, 4, 4, 5, 2, 1, 3, 3, 8, 7, 5, 7, 7, 5, 7, 8, 21, 18, 14, 19, 16, 11, 17, 19, 14, 13, 7, 11, 17, 13, 15, 18, 18, 15, 13, 17, 11, 7, 13, 14, 19 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The Bird tree is an infinite binary tree labeled with rational numbers. The root is labeled with 1. The tree enjoys the following fractal property: it can be transformed into its left subtree by first incrementing and then reciprocalizing the elements; for the right subtree interchange the order of the two steps: the elements are first reciprocalized and then incremented. Like the Stern-Brocot tree, the Bird tree enumerates all the positive rationals (A162909(n)/A162910(n)). From Yosu Yurramendi, Jul 11 2014: (Start) If the terms (n>0) are written as an array (left-aligned fashion) with rows of length 2^m, m = 0,1,2,3,... 1, 2, 1, 3, 3,1, 2, 5, 4,4, 5,2,1, 3, 3, 8, 7,5, 7,7,5, 7, 8, 3, 3,1,2, 5,4, 4, 5, 13,11,9,12,9,6,10,11,11,10,6,9,12,9,11,13,5,4,4,5,2,1,3,3,8,7,5,7,7,5,7,8, then the sum of the m-th row is 3^m (m = 0,1,2,), each column k is a Fibonacci sequence. If the rows are written in a right-aligned fashion:                                                                         1,                                                                       2,1,                                                                   3,3,1,2,                                                           5,4,4,5,2,1,3,3,                                           8,7,5,7,7,5,7,8,3,3,1,2,5,4,4,5, 13,11,9,12,9,6,10,11,11,10,6,9,12,9,11,13,5,4,4,5,2,1,3,3,8,7,5,7,7,5,7,8, then each column k also is a Fibonacci sequence. The Fibonacci sequences of both triangles are equal except the first terms of second triangle. If the sequence is considered by blocks of length 2^m, m = 0,1,2,..., the blocks of this sequence are the reverses of blocks of A162909 ( a(2^m+k) = A162909(2^(m+1)-1-k), m = 0,1,2,..., k = 0,1,2,...,2^m-1). (End) LINKS R. Hinze, Functional pearls: the bird tree, J. Funct. Programming 19 (2009), no. 5, 491-508. FORMULA a(2^m+k) = a(2^m-k-1) + a(2^(m-1)+k), a(2^m+2^(m-1)+k) = a(2^m-k-1), a(1) = 1, m=0,1,2,3,..., k=0,1,...,2^(m-1)-1. - Yosu Yurramendi, Jul 11 2014 If k is odd a(A080675(n)*2^m+k) = A268087(2^m+k), if k is even a(A136412(2^m+k+1)*2^m+k) = A268087(2^m+k), m >= 0, 0 <= k < 2^m, n > 0. a(A081254(n)) = 1, n > 0. - Yosu Yurramendi, Feb 21 2017 EXAMPLE The first four levels of the Bird tree: [1/1] [1/2, 2/1] [2/3, 1/3, 3/1, 3/2], [3/5, 3/4, 1/4, 2/5, 5/2, 4/1, 4/3, 5/3]. PROG (Haskell) import Ratio; bird :: [Rational]; bird = branch (recip . succ) (succ . recip) 1; branch f g a = a : branch f g (f a) \/ branch f g (g a); (a : as) \/ bs = a : (bs \/ as); a162909 = map numerator bird; a162910 = map denominator bird (R) blocklevel <- 6 # arbitrary a <- 1 for(m in 1:blocklevel) for(k in 0:(2^(m-1)-1)){ a[2^m+k]         = a[2^m-k-1] + a[2^(m-1)+k] a[2^m+2^(m-1)+k] = a[2^m-k-1] } a # Yosu Yurramendi, Jul 11 2014 CROSSREFS This sequence is the composition of A162912 and A059893: a(n) = A162912(A059893(n)). This sequence is a permutation of A002487(n+2). Sequence in context: A134840 A229742 A126572 * A098975 A309213 A127121 Adjacent sequences:  A162907 A162908 A162909 * A162911 A162912 A162913 KEYWORD easy,frac,nonn AUTHOR Ralf Hinze (ralf.hinze(AT)comlab.ox.ac.uk), Aug 05 2009 STATUS approved

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Last modified May 26 08:39 EDT 2020. Contains 334620 sequences. (Running on oeis4.)