OFFSET
0,3
PROG
(PARI) {a(n)=local(L, M=matrix(n+1, n+1, m, j, if(m>=j, if(m==j, 1, if(m==j+1, -3*j, polcoeff(1/sum(i=0, m-j, prod(r=0, i-1, 3*r+1)*x^i)+O(x^m), m-j)))))^-1); L=sum(i=1, #M, (-1)^(i-1)*(M-M^0)^i/i); return(if(n<0, 0, L[n+1, 1]/3))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, May 30 2005
STATUS
approved