OFFSET
0,2
COMMENTS
Here, F_{6a} is the hypergeometric function F(1/3, 1/2; 1; 12*sqrt(-3)/t_{6a}). The definition given on page 23 in the linked manuscript has a minor typo where "t_{3A}" should be "t_{6a}". - Robin Visser, Jul 31 2023
LINKS
Masao Koike, Modular forms on non-compact arithmetic triangle groups, Unpublished manuscript [Extensively annotated with OEIS A-numbers by N. J. A. Sloane, Feb 14 2021. Sloane wrote 2005 on the first page but the internal evidence suggests 1997.] See page 31.
PROG
(Sage)
def a(n):
if n==0: return 1
theta2 = sum([1]+[2*x^(k^2/2) for k in range(1, n+1)])
theta3 = sum([2*x^((k^2 + k + 1/4)/2) for k in range(n)])
phix = theta2(x=x^4)*theta2(x=x^12) + theta3(x=x^4)*theta3(x=x^12)
phiy = theta2(x=x^4)*theta3(x=x^12) + theta3(x=x^4)*theta2(x=x^12)
f = (phiy*(phix^2 - phiy^2)*phix*(phix^2 - 9*phiy^2)*(phix^2 + 3*phiy^2)^5)/2
return f.taylor(x, 0, n+1).coefficient(x^(n+1/2)) # Robin Visser, Jul 31 2023
CROSSREFS
KEYWORD
sign
AUTHOR
Robert C. Lyons, Feb 15 2021
EXTENSIONS
More terms from Robin Visser, Jul 31 2023
STATUS
approved