OFFSET
1,2
FORMULA
G.f. satisfies: A(-A(-x)) = x.
PROG
(PARI) {a(n)=local(A, B, F); A=x+x^2+x*O(x^n); if(n<1, 0, for(i=0, n, F=x+2*A^2; B=serreverse(A); A=(A+subst(B, x, F))/2); polcoeff(x+2*A^2, n, x))}
CROSSREFS
KEYWORD
sign
AUTHOR
Paul D. Hanna, May 21 2005
STATUS
approved