

A258491


Number of words of length 2n such that all letters of the quaternary 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



14, 300, 4400, 55692, 657370, 7488228, 83752760, 928406556, 10254052556, 113186465340, 1250820198264, 13852280754980, 153813849202674, 1712835575525140, 19129590267619304, 214261857777632700, 2406509409480345364, 27100348605141932540, 305944173898725745944
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OFFSET

4,1


LINKS

Alois P. Heinz, Table of n, a(n) for n = 4..900


FORMULA

a(n) ~ 12^n / (8*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)*(nj)*(k1)^j, j=0..n1))
end:
T:= (n, k)> add((1)^i*A(n, ki)/(i!*(ki)!), i=0..k):
a:= n> T(n, 4):
seq(a(n), n=4..25);


CROSSREFS

Column k=4 of A256117.
Sequence in context: A186376 A034834 A276699 * A251220 A205619 A034912
Adjacent sequences: A258488 A258489 A258490 * A258492 A258493 A258494


KEYWORD

nonn


AUTHOR

Alois P. Heinz, May 31 2015


STATUS

approved



