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A230204 Expansion of phi(-x) * f(x^3, x^5) in powers of x where phi(), f() are Ramanujan theta functions. 1
1, -2, 0, 1, 0, 1, -2, 2, 0, 0, 0, 0, -2, 0, -1, -2, 2, 0, 3, 0, 0, 2, 2, -2, 0, -2, 0, -2, -2, 0, 0, 0, 0, 1, 0, 0, 2, 2, 0, 1, -2, 2, -2, 0, 0, 0, 0, 0, -2, 0, 2, 0, -2, 0, 0, 2, 0, 0, -2, 0, 1, -2, 0, -2, 2, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 2, 2, -2, 2, 0, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

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

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..1000

M. Somos, Introduction to Ramanujan theta functions

Eric Weisstein's World of Mathematics, Ramanujan Theta Functions

FORMULA

Euler transform of period 16 sequence [ -2, -1, -1, -1, -1, -2, -2, -2, -2, -2, -1, -1, -1, -1, -2, -2, ...].

a(n) = A030204(2*n).

EXAMPLE

G.f. = 1 - 2*x + x^3 + x^5 - 2*x^6 + 2*x^7 - 2*x^12 - x^14 - 2*x^15 + ...

G.f. = q - 2*q^17 + q^49 + q^81 - 2*q^97 + 2*q^113 - 2*q^193 - q^225 + ...

MATHEMATICA

a[ n_] := SeriesCoefficient[ EllipticTheta[ 4, 0, q] QPochhammer[ -q^3, q^8] QPochhammer[ -q^5, q^8] QPochhammer[ q^8], {q, 0, n}];

PROG

(PARI) {a(n) = local(m, j); if( n<0, 0, m = 16*n + 1; sum( k=0, sqrtint(m \ 4), if( issquare(m - 16*k^2, &j), if( k==0, 1, 2) * (-1)^k * ((j%8)==1 || (j%8==7)))))}

CROSSREFS

Cf. A030204.

Sequence in context: A090239 A165276 A035698 * A325592 A161502 A279628

Adjacent sequences:  A230201 A230202 A230203 * A230205 A230206 A230207

KEYWORD

sign

AUTHOR

Michael Somos, Oct 11 2013

STATUS

approved

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Last modified March 31 14:24 EDT 2020. Contains 333151 sequences. (Running on oeis4.)