OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..200
FORMULA
a(n) = A183134(n,n).
a(n) ~ exp(-1) * 4^n * n^(n-5/2) / sqrt(Pi). - Vaclav Kotesovec, Oct 15 2014
a(n) = A294491(n) / n for n>0, a(0) = 1. - Alois P. Heinz, Oct 31 2017
EXAMPLE
a(2) = 3: aaaa, aabb, abba (with alphabet {a,b}).
MAPLE
a:= n->`if`(n=0, 1, add(binomial(2*n, j)*(n-j)*(n-1)^j, j=0..n-1)/n):
seq(a(n), n = 0..20);
MATHEMATICA
Flatten[{1, 1, Table[Sum[Binomial[2*n, j]*(n-j)*(n-1)^j, {j, 0, n-1}]/n, {n, 2, 20}]}] (* Vaclav Kotesovec, Oct 15 2014 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 15 2014
STATUS
approved