A229893 - OEIS

A229893

Expansion of q^2 * f(-q) * f(-q^4) * f(-q^16) * f(-q^2, -q^14) in powers of q where f() is a Ramanujan theta function.

1, -1, -2, 1, 0, 2, 2, -1, -2, -1, 2, -1, -2, 0, 0, 0, 1, 3, 2, -2, 2, -6, -4, 3, 0, 4, 0, 3, 2, 0, -4, 0, -2, -2, 0, -3, 0, 2, 0, 0, 4, -5, -2, 1, 6, 0, 4, 0, -3, 2, 2, 5, -8, 2, 4, -6, 0, 3, -4, -9, -8, 0, 8, 0, -2, -5, 4, 6, 0, 10, -2, 4, 6, -3, -6, 2, -2

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OFFSET

2,3

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 = 2..2500

Michael Somos, Introduction to Ramanujan theta functions

Eric Weisstein's World of Mathematics, Ramanujan Theta Functions

FORMULA

Euler transform of period 16 sequence [-1, -2, -1, -2, -1, -1, -1, -2, -1, -1, -1, -2, -1, -2, -1, -4, ...].

a(2^n) = A108520(n-1). a(16*n + 1) = a(16*n + 15) = 0. -2 * a(n) = A229502(2*n). a(8*n) = 2 * A229502(n).

EXAMPLE

G.f. = q^2 - q^3 - 2*q^4 + q^5 + 2*q^7 + 2*q^8 - q^9 - 2*q^10 - q^11 + ...

MATHEMATICA

a[ n_] := SeriesCoefficient[ q^2 QPochhammer[ q^2, q^16] QPochhammer[ q^14, q^16] QPochhammer[ q^16]^2 QPochhammer[ q] QPochhammer[ q^4], {q, 0, n}]

PROG

(PARI) {a(n) = local(A, m); if( n<2, 0, n-=2; A = x * O(x^n); polcoeff( eta(x + A) * eta(x^4 + A) * eta(x^16 + A) * sum( k=0, n\2, if( issquare( 16*k + 9, &m), (-1)^k * x^(2*k), 0), A), n))}

(Sage) CuspForms( Gamma1(16), 2, prec=79).1

(Magma) Basis( CuspForms( Gamma1(16), 2), 79)[2];

CROSSREFS

Cf. A108520, A229502.

Sequence in context: A255648 A112848 A374366 * A317683 A366371 A198727

Adjacent sequences: A229890 A229891 A229892 * A229894 A229895 A229896

KEYWORD

sign,changed

AUTHOR

Michael Somos, Oct 02 2013

STATUS

approved