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

7,1

Links

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

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);

Crossrefs

Column k=7 of A256311.

Sequence in context: A031586 A116335 A115440 * A031646 A252556 A028538

Adjacent sequences: A321034 A321035 A321036 * A321038 A321039 A321040

Keyword

nonn

Author

Alois P. Heinz, Oct 26 2018

Status

approved