OFFSET
0,1
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
Eric Weisstein's World of Mathematics, Quintuple Product Identity
FORMULA
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.
G.f.: Sum_{k in Z} x^(12*k^2 + 5*k) - x^(12*k^2 + 11*k + 2).
a(n) = -A010815(2*n + 1).
EXAMPLE
1 - x^2 - x^3 + x^7 + x^17 - x^25 - x^28 + x^38 + x^58 - x^72 - x^77 + x^93 + ...
q^25 - q^121 - q^169 + q^361 + q^841 - q^1225 - q^1369 + q^1849 + q^2809 + ...
MATHEMATICA
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 *)
a[ n_] := SeriesCoefficient[ (QPochhammer[ -q] - QPochhammer[ q]) / 2, {q, 0, 2 n + 1}]; (* Michael Somos, Nov 09 2014 *)
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 *)
PROG
(PARI) {a(n) = local(m); if( issquare( 48*n + 25, &m), kronecker( 6, m), 0)};
(PARI) {a(n) = local(m); if( n<0, 0, m = 2*n + 1; - polcoeff( eta( x + x * O(x^m)), m))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 22 2012
STATUS
approved