OFFSET
0,1
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..2500
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
EXAMPLE
G.f. = 1 + x - x^3 + x^7 + x^8 + x^14 - x^20 - x^29 + x^31 + x^42 - x^52 - x^66 + ...
G.f. = q + q^121 - q^361 + q^841 + q^961 + q^1681 - q^2401 - q^3481 + q^3721 + ...
MATHEMATICA
f[x_, y_]:= QPochhammer[-x, x*y]*QPochhammer[-y, x*y]*QPochhammer[x*y, x*y]; CoefficientList[Series[f[-x^29, x^31] + x*f[-x^19, x^41] - x^3*f[-x^11, x^49] + x^7*f[x, -x^59], {x, 0, 50}], x] (* G. C. Greubel, Aug 11 2018 *)
PROG
(PARI) {a(n) = local(m); if( issquare( 120*n + 1, &m), (-1)^(m \ 40 + m \ 12))}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Feb 28 2012
STATUS
approved