cp's OEIS Frontend

A258495 Number of words of length 2n such that all letters of the octonary 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 A258495 #7 Jun 01 2015 15:22:02
%S A258495 1430,143208,8488440,389948856,15390120042,549818906780,
%T A258495 18329867191350,581350326663600,17769492060922914,528200606751594392,
%U A258495 15368894406877386408,439845149792754810984,12426477142114470011642,347532158068343623121916,9642227504194296532321086
%N A258495 Number of words of length 2n such that all letters of the octonary 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 A258495 Alois P. Heinz, <a href="/A258495/b258495.txt">Table of n, a(n) for n = 8..650</a>
%F A258495 a(n) ~ 28^n / (25920*sqrt(Pi)*n^(3/2)). - _Vaclav Kotesovec_, Jun 01 2015
%p A258495 A:= proc(n, k) option remember; `if`(n=0, 1, k/n*
%p A258495       add(binomial(2*n, j)*(n-j)*(k-1)^j, j=0..n-1))
%p A258495     end:
%p A258495 T:= (n, k)-> add((-1)^i*A(n, k-i)/(i!*(k-i)!), i=0..k):
%p A258495 a:= n-> T(n, 8):
%p A258495 seq(a(n), n=8..25);
%Y A258495 Column k=8 of A256117.
%K A258495 nonn
%O A258495 8,1
%A A258495 _Alois P. Heinz_, May 31 2015