A328788 Expansion of psi(x^6)^5/psi(-x^3) * (f(-x)/f(-x^4))^3 in powers of x where psi(), f() are Ramanujan theta functions.

0, 0, 0, 1, -3, 0, 6, 0, -9, 4, 0, 0, 3, 0, 0, 6, -21, 0, 24, 0, -18, 8, 0, 0, -3, 0, 0, 13, -24, 0, 36, 0, -45, 12, 0, 0, 21, 0, 0, 14, -54, 0, 48, 0, -36, 24, 0, 0, -15, 0, 0, 18, -42, 0, 78, 0, -72, 20, 0, 0, 18, 0, 0, 32, -93, 0, 72, 0, -54, 24, 0, 0, 15

Offset

0,5

Comments

Number 125 of the 126 eta-quotients listed in Table 1 of Williams 2012.

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

G.f. is a period 1 Fourier series which satisfies f(-1 / (12 t)) = 144 (t/i)^2 g(t) where q = exp(2 Pi i t) and g() is the g.f. for A329651.

Links

Antti Karttunen, Table of n, a(n) for n = 0..16383

Antti Karttunen, Data supplement: n, a(n) computed for n = 0..65537

Michael Somos, Introduction to Ramanujan theta functions

Eric Weisstein's World of Mathematics, Ramanujan Theta Functions

K. S. Williams, Fourier series of a class of eta quotients, Int. J. Number Theory 8 (2012), no. 4, 993-1004.

Formula

Euler transform of period 12 sequence [-3, -3, -2, 0, -3, 2, -3, 0, -2, -3, -3, -4, ...].

Expansion of phi(-x^3) * f(-x^2, -x^10)^6 / f(x, x^5)^3 in powers of x where phi(), f(,) are Ramanujan theta functions.

Expansion of eta(q)^3 * eta(q^12)^9 / (eta(q^3) * eta(q^4)^3 * eta(q^6)^4) in powers of q.

G.f.: x^3 * Product_{n>=1} (1 - x^(3*n))^4 * (1 + x^n)^2 * (1 + x^(2*n))^6 * (1 - x^n + x^(2*n))^5 * (1 - x^(2*n) + x^(4*n))^9.

a(n) = s(n/3) - 3*s(n/4) + 3*s(n/6) - s(n/12) if n>0 where s(x) = sum of divisors of x for integer x else 0.

a(2*n + 1) = -3 * A229615(n). a(6*n + 1) = a(6*n + 5) = 0. a(6*n + 3) = A008438(n).

Example

G.f. = x^3 - 3*x^4 + 6*x^6 - 9*x^8 + 4*x^9 + 3*x^12 + 6*x^15 + ...

Mathematica

a[ n_] := SeriesCoefficient[ 2^(-9/2) x^(-15/4) (EllipticTheta[ 2, 0, x^6]^5 / EllipticTheta[ 2, Pi/4, x^3]) (QPochhammer[ x^2] / QPochhammer[ x^8])^3 , {x, 0, n}] // PowerExpand;

Prog

(PARI) {a(n) = my(s = x -> if(frac(x), 0, sigma(x))); if( n<3, 0, s(n/3) - 3*s(n/4) + 3*s(n/6) - s(n/12))};
(PARI) {a(n) = my(A); n-=3; if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^3 * eta(x^12 + A)^9 / (eta(x^3 + A) * eta(x^4 + A)^3 * eta(x^6 + A)^4), n))};
(Magma) A := Basis( ModularForms( Gamma0(12), 2), 72); A[4] - 3*A[5];

Crossrefs

Cf. A000203, A008438, A229615, A329651.

Sequence in context: A161829 A290705 A115456 * A007386 A007385 A352610

Adjacent sequences: A328785 A328786 A328787 * A328789 A328790 A328791

Keyword

sign

Author

Michael Somos, Oct 28 2019

Status

approved