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 A157679 Number of subtrees of a complete binary tree 0
 0, 1, 2, 4, 6, 9, 15, 25, 35, 49, 70, 100, 160, 256, 416, 676, 936, 1296, 1800, 2500, 3550, 5041, 7171, 10201, 16261, 25921, 41377, 66049, 107169, 173889, 282309, 458329, 634349 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Take the complete binary tree with n labeled nodes. Here is a poor picture of the tree with 6 nodes: .......R ...../...\ ..../.....\ ...o.......o ../.\...../ .o...o...o Then the number of rooted subtrees of the graph is a(n). LINKS A. V. Aho and N. J. A. Sloane, Some doubly exponential sequences, Fib. Quart., 11 (1973), 429-437. FORMULA a(0) = 0, a(1) = 1 a(n) = 1 + a(floor((n-1)/2) + a(ceiling((n-1)/2) + a(floor((n-1)/2)*a(ceiling((n-1)/2) = (1+a(floor((n-1)/2))*(1+a(ceiling((n-1)/2)) If b(n) is sequence A005468, then a(n)=b(n+1)-1. From the definition of A005468, a(n) = b(floor((n+1)/2)*b(ceiling((n+1)/2). So for every odd n a(n) is a square: a(2n-1)=b(n)^2. If c(n) is sequence A004019, then c(n)=a(2^n-1). A004019 (and Aho and Sloane) give a closed formula for c(n) that translates to a(n) = nearest integer to b^((n+1)/2) - 1" where b = 2.25851...; the formula gives the asymptotic behavior of this sequence, however it does not compute the correct values for a(n) unless n+1 is a power of two. EXAMPLE For example, for n = 3 the a(3) = 4 subtrees are: R...R...R......R .../.....\..../.\ ..o.......o..o...o MATHEMATICA a[0] = 0; a[1] = 1; a[n_?EvenQ] := a[n] = (1 + a[n/2 - 1])*(1 + a[n/2]); a[n_?OddQ] := a[n] = (1 + a[(n-1)/2])^2; Table[a[n], {n, 0, 32}] (* From Jean-François Alcover, Oct 19 2011 *) CROSSREFS Cf. A004019, A005468 Sequence in context: A127740 A024787 A076922 * A057602 A171646 A006498 Adjacent sequences:  A157676 A157677 A157678 * A157680 A157681 A157682 KEYWORD easy,nice,nonn AUTHOR Paolo Bonzini, Mar 04 2009, Mar 09 2009 STATUS approved

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