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.

55, 1820, 42228, 859180, 16482191, 307525140, 5665789450, 103894641488, 1904303218783, 34970000393120, 644169388645755, 11909742222826580, 221052371765382039, 4118875328648495940, 77037256813624307478, 1446046251457910719288, 27235202791173787953695

Offset: 4

Author

Alois P. Heinz, Oct 26 2018

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 4..770

Crossrefs

Column k=4 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))(4):
seq(a(n), n=4..25);