cp's OEIS Frontend

A258493 Number of words of length 2n such that all letters of the senary 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 A258493 #7 Jun 01 2015 15:14:49
%S A258493 132,7007,231868,6191808,146698848,3229298919,67773956250,
%T A258493 1377513928505,27389291758920,536341475466069,10391807506431956,
%U A258493 199869644353809760,3824918464184384952,72954292150964887751,1388571904028052188458,26397789023379585277557
%N A258493 Number of words of length 2n such that all letters of the senary 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 A258493 Alois P. Heinz, <a href="/A258493/b258493.txt">Table of n, a(n) for n = 6..750</a>
%F A258493 a(n) ~ 20^n / (384*sqrt(Pi)*n^(3/2)). - _Vaclav Kotesovec_, Jun 01 2015
%p A258493 A:= proc(n, k) option remember; `if`(n=0, 1, k/n*
%p A258493       add(binomial(2*n, j)*(n-j)*(k-1)^j, j=0..n-1))
%p A258493     end:
%p A258493 T:= (n, k)-> add((-1)^i*A(n, k-i)/(i!*(k-i)!), i=0..k):
%p A258493 a:= n-> T(n, 6):
%p A258493 seq(a(n), n=6..25);
%Y A258493 Column k=6 of A256117.
%K A258493 nonn
%O A258493 6,1
%A A258493 _Alois P. Heinz_, May 31 2015