OFFSET
4,2
PROG
(PARI) {a(n)=local(F=x+x^2, G=x+x*O(x^(n+1))); if(n<3, 0, for(i=1, n-2, G=subst(F, x, G)); return(polcoeff(G, n+0, x)))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Sep 18 2006
STATUS
approved