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A321036
Number of words of length 3n such that all letters of the senary 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.
2
1428, 122094, 6577494, 288152280, 11263151718, 410775452736, 14329297278912, 485304167135463, 16110681501126303, 527563895500327686, 17115058041339933282, 551744934193726332360, 17712851478876415128930, 567151641983253355425936, 18132410018268844312487352
OFFSET
6,1
LINKS
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))(6):
seq(a(n), n=6..25);
CROSSREFS
Column k=6 of A256311.
Sequence in context: A260283 A156853 A292704 * A094230 A353080 A238136
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 26 2018
STATUS
approved