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A321040
Number of words of length 3n such that all letters of the denary 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
1430715, 385671000, 59757446980, 7005490433656, 691555233881785, 60757817462444531, 4909804407096952946, 372791285261732999200, 26986460830582840320825, 1882051044395835159556710, 127426007577261157375345878, 8424538202077517861490125956
OFFSET
10,1
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))(10):
seq(a(n), n=10..25);
CROSSREFS
Column k=10 of A256311.
Sequence in context: A054852 A242824 A204288 * A345614 A346331 A345624
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 26 2018
STATUS
approved