%I #12 Feb 16 2025 08:33:18
%S 1,0,-1,-1,0,0,0,1,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,-1,0,0,-1,0,0,0,
%T 0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,
%U 0,0,0,0,0,0,-1,0,0,0,0,-1,0,0,0,0,0,0,0
%N Expansion of f(x^7, x^17) - x^2 * f(x, x^23) in powers of x where f(,) is Ramanujan's two-variable theta function.
%C Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700).
%C This is an example of the quintuple product identity in the form f(a*b^4, a^2/b) - (a/b) * f(a^4*b, b^2/a) = f(-a*b, -a^2*b^2) * f(-a/b, -b^2) / f(a, b) where a = x^5, b = x^3.
%H G. C. Greubel, <a href="/A218173/b218173.txt">Table of n, a(n) for n = 0..1000</a>
%H Michael Somos, <a href="/A010815/a010815.txt">Introduction to Ramanujan theta functions</a>
%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RamanujanThetaFunctions.html">Ramanujan Theta Functions</a>
%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/QuintupleProductIdentity.html">Quintuple Product Identity</a>
%F Expansion of f(x, x^7) * chi(-x) in powers of x where f(,) is Ramanujan's two-variable theta function and chi() is a Ramanujan theta function.
%F G.f.: Sum_{k in Z} x^(12*k^2 + 5*k) - x^(12*k^2 + 11*k + 2).
%F a(n) = -A010815(2*n + 1).
%e 1 - x^2 - x^3 + x^7 + x^17 - x^25 - x^28 + x^38 + x^58 - x^72 - x^77 + x^93 + ...
%e q^25 - q^121 - q^169 + q^361 + q^841 - q^1225 - q^1369 + q^1849 + q^2809 + ...
%t a[ n_] := If[ n < 0, 0, If[ OddQ[ DivisorSigma[ 0, 48 n + 25]], JacobiSymbol[ 6, Sqrt[48 n + 25]], 0]]; (* _Michael Somos_, Nov 09 2014 *)
%t a[ n_] := SeriesCoefficient[ (QPochhammer[ -q] - QPochhammer[ q]) / 2, {q, 0, 2 n + 1}]; (* _Michael Somos_, Nov 09 2014 *)
%t a[ n_] := SeriesCoefficient[ QPochhammer[ q] (QPochhammer[ q^2]^3 / QPochhammer[ q]^2/ QPochhammer[ q^4] - 1) / 2, {q, 0, 2 n + 1}]; (* _Michael Somos_, Nov 09 2014 *)
%o (PARI) {a(n) = local(m); if( issquare( 48*n + 25, &m), kronecker( 6, m), 0)};
%o (PARI) {a(n) = local(m); if( n<0, 0, m = 2*n + 1; - polcoeff( eta( x + x * O(x^m)), m))};
%Y Cf. A010815, A069911.
%K sign,changed
%O 0,1
%A _Michael Somos_, Oct 22 2012