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A258493
Number of words of length 2n 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 doublets into the initially empty word.
2
132, 7007, 231868, 6191808, 146698848, 3229298919, 67773956250, 1377513928505, 27389291758920, 536341475466069, 10391807506431956, 199869644353809760, 3824918464184384952, 72954292150964887751, 1388571904028052188458, 26397789023379585277557
OFFSET
6,1
LINKS
FORMULA
a(n) ~ 20^n / (384*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, 6):
seq(a(n), n=6..25);
CROSSREFS
Column k=6 of A256117.
Sequence in context: A035837 A335857 A270411 * A184893 A035818 A258394
KEYWORD
nonn
AUTHOR
Alois P. Heinz, May 31 2015
STATUS
approved