A257900 Expansion of 1/2 - (phi(-q)^2 + phi(-q^9)^2) / 4 in powers of q where phi() is a Ramanujan theta function.

1, -1, 0, -1, 2, 0, 0, -1, 2, -2, 0, 0, 2, 0, 0, -1, 2, -2, 0, -2, 0, 0, 0, 0, 3, -2, 0, 0, 2, 0, 0, -1, 0, -2, 0, -2, 2, 0, 0, -2, 2, 0, 0, 0, 4, 0, 0, 0, 1, -3, 0, -2, 2, 0, 0, 0, 0, -2, 0, 0, 2, 0, 0, -1, 4, 0, 0, -2, 0, 0, 0, -2, 2, -2, 0, 0, 0

Offset

1,5

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 = 1..2500

Michael Somos, Introduction to Ramanujan theta functions, 2019.

Eric Weisstein's World of Mathematics, Ramanujan Theta Functions.

Formula

Expansion of 1/2 - (eta(q)^4 * eta(q^18)^2 + eta(q^2)^2 * eta(q^9)^4) / (2 * eta(q^2) * eta(q^18))^2 in powers of q. - Michael Somos, Jun 02 2015

a(n) is multiplicative with a(2^e) = -1 if e>0, a(3^e) = 1 + (-1)^e if e>0, a(p^e) = (1 + (-1)^e) / 2 if p == 3 (mod 4), a(p^e) = e+1 if p == 1 (mod 4).

a(3*n + 1) = A258277(n). a(3*n + 2) = - A258278(n). a(9*n) = 2 * A113652(n). a(9*n + 3) = a(9*n + 6) = 0.

-2 * a(n) = A258322(n) unless n = 0 or n == 2 (mod 3).

Sum_{k=1..n} abs(a(k)) ~ (5*Pi/18) * n. - Amiram Eldar, Jan 29 2024

Example

G.f. = q - q^2 - q^4 + 2*q^5 - q^8 + 2*q^9 - 2*q^10 + 2*q^13 - q^16 + ...

Mathematica

a[ n_] := SeriesCoefficient[ 1/2 - (EllipticTheta[ 4, 0, q]^2 + EllipticTheta[ 4, 0, q^9]^2) / 4, {q, 0, n}]; (* Michael Somos, Jun 02 2015 *)

Prog

(PARI) {a(n) = my(A, p, e); if( n<1, 0, A = factor(n); prod( k=1, matsize(A)[1], [p, e] = A[k, ]; if( p==2, -1, p%4==3, if( p>3, 1, 2) * (1-e%2), e+1)))};

Crossrefs

Cf. A113652, A258277, A258278, A258322.

Cf. A000122, A000700, A010054, A121373.

Sequence in context: A303065 A325406 A360675 * A362426 A039971 A205593

Adjacent sequences: A257897 A257898 A257899 * A257901 A257902 A257903

Keyword

sign,mult

Author

Michael Somos, May 25 2015

Status

approved