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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 (list; graph; refs; listen; history; text; internal format)
OFFSET

6,1

LINKS

Alois P. Heinz, Table of n, a(n) for n = 6..750

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

Adjacent sequences:  A258490 A258491 A258492 * A258494 A258495 A258496

KEYWORD

nonn

AUTHOR

Alois P. Heinz, May 31 2015

STATUS

approved

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Last modified January 24 22:09 EST 2021. Contains 340414 sequences. (Running on oeis4.)