A113430 Expansion of f(-x, -x^2) * f(-x^10, -x^20) / f(-x^2, -x^8) in powers of x where f(, ) is Ramanujan's general theta function.

1, -1, 0, -1, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, -1, 0, 0, 0, 0, 0, -1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0

Offset

0,1

Comments

Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700).

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^4, b = x.

References

George E. Andrews, Richard Askey and Ranjan Roy, Special Functions, Cambridge University Press, 1999.

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^7, x^8) - x * f(x^2, x^13) in power of x.

Expansion of G(x^2) * f(-x) where G() is the g.f. of A003114.

Euler transform of period 10 sequence [ -1, 0, -1, -1, -1, -1, -1, 0, -1, -1, ...].

|a(n)| is the characteristic function of the numbers in A093722.

The exponents in the q-series q * A(q^120) are the square of the numbers in A057538.

G.f.: Prod_{k>0} (1 - x^k) / ((1 - x^(10*k - 2)) * (1 - x^(10*k - 8))) = Sum_{k in Z} x^((15*k^2 + k) / 2) - x^((15*k^2 - 11*k + 2) / 2).

A(q^2) = 1 + Sum_{n >= 0} q^(n^2) * Product_{k >= 2*n+1} 1 - q^k = 1 - q^2 - q^6 + q^14 + q^16 - q^28 - q^40 + + - - . See Andrews et al., p. 591, Exercise 6(a). - Peter Bala, Dec 22 2024

Example

G.f. = 1 - x - x^3 + x^7 + x^8 - x^14 - x^20 + x^29 + x^31 - x^42 - x^52 + ...
G.f. = q - q^121 - q^361 + q^841 + q^961 - q^1681 - q^2401 + q^3481 + q^3721 + ...

Mathematica

a[ n_] := SeriesCoefficient[ QPochhammer[ x] / (QPochhammer[ x^2, x^10] QPochhammer[ x^8, x^10]), {x, 0, n}]; (* Michael Somos, Jan 06 2016 *)

Prog

(PARI) {a(n) = my(m); if( n<0 || !issquare( n*120 + 1, &m) || 1!=gcd(m, 30), 0, (-1)^(m%30\10))};
(PARI)  {a(n) = if( n<0, 0, polcoeff( prod( k=1, n, 1 - x^k * [1, 1, 0, 1, 1, 1, 1, 1, 0, 1][k%10 + 1], 1 + x * O(x^n)), n))};

Crossrefs

Cf. A003114, A010815, A057538, A093722.

Sequence in context: A207710 A207735 A208546 * A214529 A113681 A356163

Adjacent sequences: A113427 A113428 A113429 * A113431 A113432 A113433

Keyword

sign,easy

Author

Michael Somos, Oct 31 2005

Status

approved