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A258496
Number of words of length 2n 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 doublets into the initially empty word.
2
4862, 629850, 47432550, 2728352253, 133216751525, 5829093450180, 236006398327050, 9025008152896320, 330547676678287002, 11710509049983422030, 404211829411082901714, 13667296618312167097605, 454559414725395785663741, 14918526141220986683667840
OFFSET
9,1
LINKS
FORMULA
a(n) ~ 32^n / (246960*sqrt(Pi)*n^(3/2)). - Vaclav Kotesovec, Jun 01 2015
MAPLE
A:= proc(n, k) option remember; `if`(n=0, 1, k/n*
add(binomial(2*n, j)*(n-j)*(k-1)^j, j=0..n-1))
end:
T:= (n, k)-> add((-1)^i*A(n, k-i)/(i!*(k-i)!), i=0..k):
a:= n-> T(n, 9):
seq(a(n), n=9..25);
CROSSREFS
Column k=9 of A256117.
Sequence in context: A124088 A244106 A264182 * A258397 A215549 A295442
KEYWORD
nonn
AUTHOR
Alois P. Heinz, May 31 2015
STATUS
approved