OFFSET
0,3
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 24 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Masao Koike, Modular forms on non-compact arithmetic triangle groups, Unpublished manuscript [Extensively annotated with OEIS A-numbers by N. J. A. Sloane, Feb 14 2021.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 30.
PROG
(Sage)
def a(n):
if n==0: return 0
eta = x^(1/24)*product([(1 - x^k) for k in range(1, 2*n+1)])
t6a = ((eta(x=x^2)/eta(x=x^6))^6
- 27*(eta(x=x^6)/eta(x=x^2))^6)(x=sqrt(x)) + 6*sqrt(-3)
F6a = sum([rising_factorial(1/3, k)*rising_factorial(1/2, k)/
(rising_factorial(1, k)^2)*((12*sqrt(-3))/t6a)^k for k in range(2*n+1)])
f = (1/t6a^3)*(1-6*sqrt(-3)/t6a)*F6a^12
return f.taylor(x, 0, n+1).coefficients()[n-1][0] # Robin Visser, Jul 24 2023
CROSSREFS
KEYWORD
sign
AUTHOR
Robert C. Lyons, Feb 15 2021
EXTENSIONS
More terms from Robin Visser, Jul 24 2023
STATUS
approved