cp's OEIS Frontend

A258494 Number of words of length 2n such that all letters of the septenary 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 A258494 #7 Jun 01 2015 15:19:33
%S A258494 429,32032,1447992,51647700,1605746373,45730269662,1227377689395,
%T A258494 31600247019120,789604855530855,19302666179660304,464277874912868104,
%U A258494 11032638071392824348,259801742258903758053,6076538940891732415102,141407795779400734204647
%N A258494 Number of words of length 2n such that all letters of the septenary 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 A258494 Alois P. Heinz, <a href="/A258494/b258494.txt">Table of n, a(n) for n = 7..700</a>
%F A258494 a(n) ~ 24^n / (3000*sqrt(Pi)*n^(3/2)). - _Vaclav Kotesovec_, Jun 01 2015
%p A258494 A:= proc(n, k) option remember; `if`(n=0, 1, k/n*
%p A258494       add(binomial(2*n, j)*(n-j)*(k-1)^j, j=0..n-1))
%p A258494     end:
%p A258494 T:= (n, k)-> add((-1)^i*A(n, k-i)/(i!*(k-i)!), i=0..k):
%p A258494 a:= n-> T(n, 7):
%p A258494 seq(a(n), n=7..25);
%Y A258494 Column k=7 of A256117.
%K A258494 nonn
%O A258494 7,1
%A A258494 _Alois P. Heinz_, May 31 2015