cp's OEIS Frontend

A258496 Number of words of length 2n such that all letters of the nonary alphabet occur at least once and are introduced in ascending order and which can be built by repeatedly inserting doublets into the initially empty word.

%I A258496 #7 Jun 01 2015 15:38:00
%S A258496 4862,629850,47432550,2728352253,133216751525,5829093450180,
%T A258496 236006398327050,9025008152896320,330547676678287002,
%U A258496 11710509049983422030,404211829411082901714,13667296618312167097605,454559414725395785663741,14918526141220986683667840
%N A258496 Number of words of length 2n such that all letters of the nonary alphabet occur at least once and are introduced in ascending order and which can be built by repeatedly inserting doublets into the initially empty word.
%H A258496 Alois P. Heinz, <a href="/A258496/b258496.txt">Table of n, a(n) for n = 9..650</a>
%F A258496 a(n) ~ 32^n / (246960*sqrt(Pi)*n^(3/2)). - _Vaclav Kotesovec_, Jun 01 2015
%p A258496 A:= proc(n, k) option remember; `if`(n=0, 1, k/n*
%p A258496       add(binomial(2*n, j)*(n-j)*(k-1)^j, j=0..n-1))
%p A258496     end:
%p A258496 T:= (n, k)-> add((-1)^i*A(n, k-i)/(i!*(k-i)!), i=0..k):
%p A258496 a:= n-> T(n, 9):
%p A258496 seq(a(n), n=9..25);
%Y A258496 Column k=9 of A256117.
%K A258496 nonn
%O A258496 9,1
%A A258496 _Alois P. Heinz_, May 31 2015