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A232714 Expansion of f(-x, -x^6) in powers of x where f is Ramanujan's two-variable theta function. 4
1, -1, 0, 0, 0, 0, -1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, -1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1, 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, 0, -1, 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).

LINKS

Seiichi Manyama, Table of n, a(n) for n = 0..10000

M. Somos, Introduction to Ramanujan theta functions

Eric Weisstein's World of Mathematics, Ramanujan Theta Functions

FORMULA

Euler transform of period 7 sequence [ -1, 0, 0, 0, 0, -1, -1, ...].

G.f.: Sum_{k in Z} (-1)^k * x^(k * (7*k + 5) / 2).

G.f.: Product_{k>0} (1 - x^(7*k-6)) * (1 - x^(7*k-1)) * (1 - x^(7*k)).

a(3*n + 2) = a(5*n + 2) = a(5*n + 3) = 0.

Convolution inverse of A195849.

abs(a(n)) = A274179(n). - Michael Somos, Jan 28 2017

a(n) = -(1/n)*Sum_{k=1..n} A284363(k)*a(n-k), a(0) = 1. - Seiichi Manyama, Mar 25 2017

EXAMPLE

G.f. = 1 - x - x^6 + x^9 + x^19 - x^24 - x^39 + x^46 + x^66 - x^75 - x^100 + ...

G.f. = q^25 - q^81 - q^361 + q^529 + q^1089 - q^1369 - q^2209 + q^2601 + q^3721 + ...

MATHEMATICA

a[ n_] := SeriesCoefficient[ SeriesCoefficient[ QPochhammer[ q, q^7] QPochhammer[ q^6, q^7] QPochhammer[ q^7], {q, 0, n}];

PROG

(PARI) {a(n) = my(m); if( issquare( 56*n + 25, &m), (-1)^round( m / 14), 0)};

CROSSREFS

Cf. A195849, A274179.

Sequence in context: A113052 A256432 A218171 * A274179 A194675 A117964

Adjacent sequences:  A232711 A232712 A232713 * A232715 A232716 A232717

KEYWORD

sign

AUTHOR

Michael Somos, Nov 28 2013

STATUS

approved

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Last modified October 20 22:44 EDT 2019. Contains 328291 sequences. (Running on oeis4.)