generalization of A005251, A118891

[Max Alekseyev]

On 6/26/07, Max Alekseyev <maxale at gmail.com> wrote:
> a_4(n) is missing in OEIS.
> A straightforward PARI code and initial 0th to 20th terms:
>
> { a4(n) = 1 + sum(i=1,n, sum(j=0,n-i, sum(k=0,(n-i-j)\2,
> sum(l=0,(n-i-j-2*k)\4,
> binomial(i-1,j)*binomial(i-1-j,k)*binomial(i-1-j-2*k,l)*binomial(n-i-j-2*k-3*l+1,l+1)
> )))) }
> ? vector(21,n,a2(n-1))
> %1 = [1, 2, 4, 7, 12, 21, 37, 65, 114, 200, 351, 616, 1081, 1897,
> 3329, 5842, 10252, 17991, 31572, 55405, 97229]

Ops, wrong copy-paste. Correct one:

? vector(21,n,a4(n-1))
%1 = [1, 2, 4, 8, 16, 31, 60, 116, 223, 428, 820, 1569, 3002, 5744,
10992, 21039, 40273, 77095, 147588, 282538, 540881]

Max