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A127524 Number of unordered rooted trees where each subtree from given node has the same number of nodes. 14

%I #11 Feb 25 2017 12:16:57

%S 1,1,2,3,5,6,11,12,20,25,42,43,81,82,150,192,287,288,563,564,982,1277,

%T 2182,2183,3658,3785,7108,8659,13101,13102,27827,27828,47768,61025,

%U 102355,105689,170882,170883,329651,421547,606283,606284,1193038,1193039,2158117

%N Number of unordered rooted trees where each subtree from given node has the same number of nodes.

%H Alois P. Heinz, <a href="/A127524/b127524.txt">Table of n, a(n) for n = 1..1000</a>

%F a(1) = 1; a(n+1) = Sum_{d|n} C(a(n/d) + d-1, d).

%e The tree shown below left counts, because the subtree shown on the left has 3 nodes and so does the one on the right and a similar condition holds for the subtrees. The tree shown on the right is not counted, because the subtree shown on the left has 3 nodes, while the one on the right has 4.

%e O..........O...O...O

%e |..........|....\./.

%e O...O...O..O.....O..

%e .\...\./....\....|..

%e .O...O......O...O..

%e ..\./........\./...

%e ...O..........O....

%p with(numtheory):

%p a:= proc(n) option remember; `if`(n<2, n,

%p add(binomial(a((n-1)/d)+d-1, d), d=divisors(n-1)))

%p end:

%p seq(a(n), n=1..50); # _Alois P. Heinz_, May 16 2013

%t a[1] = 1; a[n_] := a[n] = DivisorSum[n-1, Binomial[a[(n-1)/#]+#-1, #]&]; Table[a[n], {n, 1, 50}] (* _Jean-François Alcover_, Feb 25 2017 *)

%Y Cf. A000081, A127525.

%K nonn

%O 1,3

%A _Franklin T. Adams-Watters_, Jan 17 2007

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Last modified March 29 01:36 EDT 2024. Contains 371264 sequences. (Running on oeis4.)