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A207735 Expansion of f(-x^2, x^3)^2 / f(x, -x^2) in powers of x where f() is Ramanujan's two-variable theta function. 1
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 (list; graph; refs; listen; history; text; internal format)
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.

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) = f(x^5, -x^10) * f(-x^2, x^3) / f(x, -x^4) where f() is Ramanujan's two-variable theta function.

Euler transform of period 20 sequence [ -1, 0, 1, 1, 1, 0, 1, -1, -1, -2, -1, -1, 1, 0, 1, 1, 1, 0, -1, -1, ...].

G.f.: Sum_{k} (-1)^[k/2] * x^(5*k * (3*k + 1)/2) * (x^(-3*k) - x^(3*k + 1)).

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

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

a(7*n + 2) = a(7*n + 4) = a(7*n + 5) = 0. a(n) * (-1)^n = A113681(n).

EXAMPLE

1 - x + x^3 + x^7 - x^8 - x^14 + x^20 - x^29 - x^31 + x^42 - x^52 - x^66 + ...

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]; A207735[n_] := SeriesCoefficient[f[x^5, -x^10]*f[-x^2, x^3]/f[x, -x^4], {x, 0, n}]; Table[A207735[n], {n, 0, 50}] (* G. C. Greubel, Jun 18 2017 *)

PROG

(PARI) {a(n) = local(m); if( issquare( 120*n + 1, &m), (-1)^n * kronecker( 12, m), 0)}

CROSSREFS

Cf. A057538, A093722, A113681.

Sequence in context: A053866 A143259 A207710 * A208546 A113430 A214529

Adjacent sequences:  A207732 A207733 A207734 * A207736 A207737 A207738

KEYWORD

sign

AUTHOR

Michael Somos, Feb 19 2012

STATUS

approved

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Last modified June 19 15:58 EDT 2021. Contains 345144 sequences. (Running on oeis4.)