A321033 Number of words of length 3n such that all letters of the ternary alphabet occur at least once and are introduced in ascending order and which can be built by repeatedly inserting triples into the initially empty word.

12, 198, 2520, 29886, 347907, 4048966, 47431488, 560528820, 6684500548, 80402461416, 974694481735, 11899112853684, 146175947513247, 1805742311925206, 22417900292210960, 279556262002954608, 3500139320096264886, 43982137867266022438, 554494988892196104240

Offset: 3

Author

Alois P. Heinz, Oct 26 2018

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 3..889

Crossrefs

Column k=3 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))(3):
seq(a(n), n=3..25);