OFFSET
-1,3
EXAMPLE
1/x + 1 + 2*x - x^3 + 2*x^5 - 5*x^7 + 12*x^9 - 30*x^11 + 82*x^13 - ...
PROG
(PARI) {a(n) = local(A, m); if( n<-1, 0, if( n%2 == 0, n==0, A = 1 + O(x); m=1; while( 2*m <= n+1, A = sqrt(4*x + subst(A, x, x^2)); m*=2); polcoeff(A, (n+1)/2)))}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Aug 04 2011
STATUS
approved