OFFSET
-1,2
LINKS
Vaclav Kotesovec, Table of n, a(n) for n = -1..5000
I. Chen and N. Yui, Singular values of Thompson series. In Groups, difference sets and the Monster (Columbus, OH, 1993), pp. 255-326, Ohio State University Mathematics Research Institute Publications, 4, de Gruyter, Berlin, 1996.
J. H. Conway and S. P. Norton, Monstrous Moonshine, Bull. Lond. Math. Soc. 11 (1979) 308-339.
D. Ford, J. McKay and S. P. Norton, More on replicable functions, Commun. Algebra 22, No. 13, 5175-5193 (1994).
FORMULA
Expansion of Hauptmodul for Gamma_0(13)+.
Expansion of (eta(q) / eta(q^13))^2 + 13*(eta(q^13) / eta(q))^2 in powers of q. - Michael Somos, Jul 05 2012
G.f.: (1/x) * Product_{k>0} ((1 - x^k) / (1 - x^(13*k)))^2 + 13*x * Product_{k>0} ((1 - x^(13*k)) / (1 - x^k))^2. - Michael Somos, Jul 05 2012
a(n) = A034319(n) unless n=0. - Michael Somos, Jul 05 2012
a(n) ~ exp(4*Pi*sqrt(n/13)) / (sqrt(2) * 13^(1/4) * n^(3/4)). - Vaclav Kotesovec, Sep 08 2017
EXAMPLE
G.f. = 1/q - 2 + 12*q + 28*q^2 + 66*q^3 + 132*q^4 + 258*q^5 + 468*q^6 + ...
MATHEMATICA
QP = QPochhammer; A = O[q]^40; A = (QP[q + A]/QP[q^13 + A])^2; s = A + 13*(q^2/A); CoefficientList[s, q] (* Jean-François Alcover, Nov 13 2015, adapted from PARI *)
PROG
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = x * O(x^n); A = (eta(x + A) / eta(x^13 + A))^2; polcoeff( A + 13 * x^2 / A, n))}; /* Michael Somos, Jul 05 2012 */
CROSSREFS
KEYWORD
sign,easy
AUTHOR
STATUS
approved