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A321039
Number of words of length 3n such that all letters of the nonary 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
246675, 52676325, 6567205788, 627427976340, 51015901999920, 3724987212716133, 252083271295845990, 16134288197281838562, 990146359650754095405, 58830559749207291469515, 3408249740757631887365820, 193544431133535679583811150, 10816879949695374764949152976
OFFSET
9,1
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))(9):
seq(a(n), n=9..25);
CROSSREFS
Column k=9 of A256311.
Sequence in context: A251856 A146544 A237313 * A230525 A052198 A053077
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 26 2018
STATUS
approved