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.

7752, 942172, 69858360, 4110110550, 211675627240, 10018481786544, 448004505210535, 19256258184412314, 804574034878107648, 32931600573236529164, 1327621591399983233480, 52924527085674622431690, 2092267882428782767625532, 82204203906811813227807616

Offset: 7

Author

Alois P. Heinz, Oct 26 2018

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 7..627

Crossrefs

Column k=7 of A256311.

Programs

Maple

b:= (n, k)-> `if`(n=0, 1, k/n*add(binomial(3*n, j)*(n-j)*(k-1)^j, j=0..n-1)):
a:= n-> (k-> add((-1)^i*b(n, k-i)/(i!*(k-i)!), i=0..k))(7):
seq(a(n), n=7..25);