cp's OEIS Frontend

A321039 Number of words of length 3n 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 triples of identical letters into the initially empty word.

%I A321039 #4 Oct 26 2018 16:37:20
%S A321039 246675,52676325,6567205788,627427976340,51015901999920,
%T A321039 3724987212716133,252083271295845990,16134288197281838562,
%U A321039 990146359650754095405,58830559749207291469515,3408249740757631887365820,193544431133535679583811150,10816879949695374764949152976
%N A321039 Number of words of length 3n 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 triples of identical letters into the initially empty word.
%H A321039 Alois P. Heinz, <a href="/A321039/b321039.txt">Table of n, a(n) for n = 9..583</a>
%p A321039 b:= (n, k)-> `if`(n=0, 1, k/n*add(binomial(3*n, j)*(n-j)*(k-1)^j, j=0..n-1)):
%p A321039 a:= n-> (k-> add((-1)^i*b(n, k-i)/(i!*(k-i)!), i=0..k))(9):
%p A321039 seq(a(n), n=9..25);
%Y A321039 Column k=9 of A256311.
%K A321039 nonn
%O A321039 9,1
%A A321039 _Alois P. Heinz_, Oct 26 2018