Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #4 Oct 26 2018 16:37:20
%S 246675,52676325,6567205788,627427976340,51015901999920,
%T 3724987212716133,252083271295845990,16134288197281838562,
%U 990146359650754095405,58830559749207291469515,3408249740757631887365820,193544431133535679583811150,10816879949695374764949152976
%N 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 Alois P. Heinz, <a href="/A321039/b321039.txt">Table of n, a(n) for n = 9..583</a>
%p b:= (n, k)-> `if`(n=0, 1, k/n*add(binomial(3*n, j)*(n-j)*(k-1)^j, j=0..n-1)):
%p a:= n-> (k-> add((-1)^i*b(n, k-i)/(i!*(k-i)!), i=0..k))(9):
%p seq(a(n), n=9..25);
%Y Column k=9 of A256311.
%K nonn
%O 9,1
%A _Alois P. Heinz_, Oct 26 2018