A263073 Expansion of phi(-x^5) / (chi(-x) * chi(-x^15)) in powers of x where phi(), chi() are Ramanujan theta functions.

1, 1, 1, 2, 2, 1, 2, 3, 2, 4, 4, 4, 5, 6, 6, 8, 9, 9, 12, 12, 13, 16, 18, 18, 22, 24, 25, 29, 32, 34, 40, 43, 45, 52, 56, 60, 68, 74, 78, 88, 95, 101, 113, 122, 130, 145, 156, 166, 184, 198, 209, 231, 249, 264, 290, 311, 331, 361, 388, 412, 448, 480, 510, 554

Offset: 0

Author

Michael Somos, Oct 08 2015

Keywords

• nonn

Comments

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

Examples

			G.f. = 1 + x + x^2 + 2*x^3 + 2*x^4 + x^5 + 2*x^6 + 3*x^7 + 2*x^8 + 4*x^9 + ...
G.f. = q^2 + q^5 + q^8 + 2*q^11 + 2*q^14 + q^17 + 2*q^20 + 3*q^23 + 2*q^26 + ...
		

Links

• G. C. Greubel, Table of n, a(n) for n = 0..2500
• Vaclav Kotesovec, A method of finding the asymptotics of q-series based on the convolution of generating functions, arXiv:1509.08708 [math.CO], Sep 30 2015.
• Michael Somos, Introduction to Ramanujan theta functions
• Eric Weisstein's World of Mathematics, Ramanujan Theta Functions

Programs

Mathematica

a[ n_] := SeriesCoefficient[ EllipticTheta[ 4, 0, x^5] / (QPochhammer[ x, x^2] QPochhammer[ x^15, x^30]), {x, 0, n}];
nmax = 100; CoefficientList[Series[Product[(1+x^k) * (1-x^(5*k)) * (1+x^(15*k)) / (1+x^(5*k)), {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Jul 11 2016 *)

PARI

{a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^2 + A) * eta(x^5 + A)^2 * eta(x^30 + A) / (eta(x + A) * eta(x^10 + A) * eta(x^15 + A)), n))};

PARI

q='q+O('q^99); Vec(eta(q^2)*eta(q^5)^2*eta(q^30)/(eta(q)*eta(q^10)*eta(q^15))) \\ Altug Alkan, Jul 31 2018

Formula

Expansion of q^(-2/3) * eta(q^2) * eta(q^5)^2 * eta(q^30) / (eta(q) * eta(q^10) * eta(q^15)) in powers of q.

Euler transform of period 30 sequence [1, 0, 1, 0, -1, 0, 1, 0, 1, -1, 1, 0, 1, 0, 0, 0, 1, 0, 1, -1, 1, 0, 1, 0, -1, 0, 1, 0, 1, -1, ...].

a(n) ~ exp(sqrt(7*n/5)*Pi/3) / (2*sqrt(5*n)). - Vaclav Kotesovec, Jul 11 2016