OFFSET
0,3
FORMULA
G.f.: A(x) = A(x/(1-2x))/A(x/(1-x))*(1-x)/(1-2x).
PROG
(PARI) {a(n)=local(A); if(n<0, 0, A=1+x+x*O(x^n); for(k=1, n, B=subst(A, x, x/(1-x))/(1-x)+x*O(x^n); C=subst(B, x, x/(1-x))/(1-x)+x*O(x^n); A=A-A*B+C); polcoeff(A, n, x))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Nov 26 2003
STATUS
approved