cp's OEIS Frontend

A321034 Number of words of length 3n such that all letters of the quaternary alphabet occur at least once and are introduced in ascending order and which can be built by repeatedly inserting triples of identical letters into the initially empty word.

%I A321034 #4 Oct 26 2018 15:50:12
%S A321034 55,1820,42228,859180,16482191,307525140,5665789450,103894641488,
%T A321034 1904303218783,34970000393120,644169388645755,11909742222826580,
%U A321034 221052371765382039,4118875328648495940,77037256813624307478,1446046251457910719288,27235202791173787953695
%N A321034 Number of words of length 3n such that all letters of the quaternary alphabet occur at least once and are introduced in ascending order and which can be built by repeatedly inserting triples of identical letters into the initially empty word.
%H A321034 Alois P. Heinz, <a href="/A321034/b321034.txt">Table of n, a(n) for n = 4..770</a>
%p A321034 b:= (n, k)-> `if`(n=0, 1, k/n*add(binomial(3*n, j)*(n-j)*(k-1)^j, j=0..n-1)):
%p A321034 a:= n-> (k-> add((-1)^i*b(n, k-i)/(i!*(k-i)!), i=0..k))(4):
%p A321034 seq(a(n), n=4..25);
%Y A321034 Column k=4 of A256311.
%K A321034 nonn
%O A321034 4,1
%A A321034 _Alois P. Heinz_, Oct 26 2018