OFFSET
0,2
PROG
(PARI) {a(n)=local(N=matrix(n+1, n+1, m, j, if(m>=j, binomial(m-1, j-1)*binomial(m, j-1)/j))); sum(k=0, n, sum(j=0, n-k, (N^k)[n-k+1, j+1]))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Apr 14 2005
STATUS
approved