OFFSET
0,2
EXAMPLE
G.f.: A(x) = 1 + 2*x + 6*x^2 + 20*x^3 + 62*x^4 + 204*x^5 + 684*x^6 +...
where
1/A(x)^2 = 1 - 4*x + 16*x^4 - 64*x^9 + 256*x^16 - 1024*x^25 + 4096*x^36 - 16384*x^49 +...+ (-4)^n*x^(n^2) +...
PROG
(PARI) {a(n)=local(S=sum(m=0, sqrtint(n), (-4)^m*x^(m^2))+x*O(x^n)); polcoeff(S^(-1/2), n)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Jul 18 2011
STATUS
approved