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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.
2
12, 198, 2520, 29886, 347907, 4048966, 47431488, 560528820, 6684500548, 80402461416, 974694481735, 11899112853684, 146175947513247, 1805742311925206, 22417900292210960, 279556262002954608, 3500139320096264886, 43982137867266022438, 554494988892196104240
OFFSET
3,1
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))(3):
seq(a(n), n=3..25);
CROSSREFS
Column k=3 of A256311.
Sequence in context: A066230 A262845 A358363 * A048667 A115865 A291285
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 26 2018
STATUS
approved