OFFSET
-1,2
REFERENCES
J. M. Borwein and P. B. Borwein, Pi and the AGM, Wiley, 1987, p. 195.
S. Ramanujan, Modular Equations and Approximations to pi, pp. 23-39 of Collected Papers of Srinivasa Ramanujan, Ed. G. H. Hardy et al., AMS Chelsea 2000. See page 26.
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Seiichi Manyama, Table of n, a(n) for n = -1..10000 (terms -1..1000 from T. D. Noe)
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).
Yang-Hui He, John McKay, Sporadic and Exceptional, arXiv:1505.06742 [math.AG], 2015.
J. McKay and H. Strauss, The q-series of monstrous moonshine and the decomposition of the head characters, Comm. Algebra 18 (1990), no. 1, 253-278.
FORMULA
G.f. 48 + 64(g_n^(24) + g_n^(-24)) where q = e^(-Pi sqrt(n)) and g_n is Ramanujan's class invariant. - Michael Somos, Apr 20 2005
a(n) ~ exp(2*Pi*sqrt(2*n)) / (2^(3/4)*n^(3/4)). - Vaclav Kotesovec, Apr 01 2017
EXAMPLE
G.f. = 1/q + 24 + 4372*q + 96256*q^2 + 1240002*q^3 + 10698752*q^4 + ...
MATHEMATICA
a[n0_] := Module[{n=n0, A}, If[n < -1, 0, n++; A = Product[ 1 - x^(2*k-1) , {k, 1, Quotient[n+1, 2]}]^24; SeriesCoefficient[ A + x*48 + x^2*4096/A, {x, 0, n}]]]; Table[ a[n], {n, -1, 16}] (* Jean-François Alcover, Oct 16 2012, after Michael Somos *)
a[ n_] := SeriesCoefficient[ With[{A = q QPochhammer[ -q, q]^24}, -80 + (1 + 64 A)^2 / A], {q, 0, n}]; (* Michael Somos, Apr 06 2015 *)
nmax = 50; CoefficientList[Series[48*x + 4096*x^2*Product[(1 + x^k)^24, {k, 1, nmax}] + Product[1/(1 + x^k)^24, {k, 1, nmax}], {x, 0, nmax}], x] (* Vaclav Kotesovec, Apr 01 2017 *)
PROG
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = prod( k=1, (n+1)\2, 1 - x^(2*k - 1), 1 + x * O(x^n))^24; polcoeff( A + x*48 + x^2 * 4096/A, n))}; /* Michael Somos, Feb 07 2003 */
(PARI) {a(n) = my(A); if( n<-1, 0, n++; A = x * O(x^n); A = (eta(x^2 + A) / eta(x + A))^24; polcoeff( -80*x + (1 + 64 * x * A)^2 / A, n))}; /* Michael Somos, Apr 06 2015 */
CROSSREFS
KEYWORD
nonn,nice
AUTHOR
STATUS
approved