cp's OEIS Frontend

A321037 Number of words of length 3n 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 triples of identical letters into the initially empty word.

%I A321037 #4 Oct 26 2018 16:25:42
%S A321037 7752,942172,69858360,4110110550,211675627240,10018481786544,
%T A321037 448004505210535,19256258184412314,804574034878107648,
%U A321037 32931600573236529164,1327621591399983233480,52924527085674622431690,2092267882428782767625532,82204203906811813227807616
%N A321037 Number of words of length 3n 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 triples of identical letters into the initially empty word.
%H A321037 Alois P. Heinz, <a href="/A321037/b321037.txt">Table of n, a(n) for n = 7..627</a>
%p A321037 b:= (n, k)-> `if`(n=0, 1, k/n*add(binomial(3*n, j)*(n-j)*(k-1)^j, j=0..n-1)):
%p A321037 a:= n-> (k-> add((-1)^i*b(n, k-i)/(i!*(k-i)!), i=0..k))(7):
%p A321037 seq(a(n), n=7..25);
%Y A321037 Column k=7 of A256311.
%K A321037 nonn
%O A321037 7,1
%A A321037 _Alois P. Heinz_, Oct 26 2018