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A097197
Expansion of q^(-1/3) * eta(q^6)^2 / (eta(q) * eta(q^3)) in powers of q.
5
1, 1, 2, 4, 6, 9, 14, 20, 29, 42, 58, 80, 110, 148, 198, 264, 347, 454, 592, 764, 982, 1257, 1598, 2024, 2554, 3206, 4010, 5000, 6208, 7684, 9484, 11664, 14306, 17501, 21346, 25972, 31526, 38170, 46112, 55588, 66861, 80258, 96154, 114968, 137212, 163472
OFFSET
0,3
COMMENTS
Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700).
REFERENCES
N. J. Fine, Basic Hypergeometric Series and Applications, Amer. Math. Soc., 1988; p. 53, Eq. (25.95).
Srinivasa Ramanujan, The Lost Notebook and Other Unpublished Papers, Narosa Publishing House, New Delhi, 1988, p. 6, 3rd equation.
LINKS
Andrew Sills, Rademacher-Type Formulas for Restricted Partition and Overpartition Functions, Ramanujan Journal, 23 (1-3): 253-264, 2010.
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
a(n) = (-1)^n * A139135(n).
Expansion of psi(q^3) / f(-q) in powers of q where psi(), f() are Ramanujan theta functions.
Euler transform of period 6 sequence [ 1, 1, 2, 1, 1, 0, ...]. - Michael Somos, Aug 19 2006
G.f.: (Sum_{k>=0} x^(3(k^2 + k)/2)) / (Product_{k>0} 1-x^k).
G.f.: (Sum_{k>0} x^(3(k^2 - k)/2)) / ((1 - x) * (1 - x^2) ...) = Product_{k>0} (1 + x^(3*k)) * (1 - x^(6*k)) / (1 - x^k).
G.f.: Product_{k>0} (1 + x^k + x^(2*k)) * (1 + x^(3*k))^2. - Michael Somos, Apr 10 2008
a(n) ~ Pi * BesselI(1, sqrt(6*n+2)*Pi/3) / (2*sqrt(18*n+6)) ~ exp(Pi*sqrt(2*n/3)) / (2^(9/4) * 3^(3/4) * n^(3/4)) * (1 + (-9/(8*Pi) + Pi/3)/sqrt(6*n) + (-5/16 - 45/(256*Pi^2) + Pi^2/108)/n). - Vaclav Kotesovec, Nov 14 2015, extended Jan 09 2017
EXAMPLE
G.f. = 1 + x + 2*x^2 + 4*x^3 + 6*x^4 + 9*x^5 + 14*x^6 + 20*x^7 + 29*x^8 + 42*x^9 + ...
G.f. = q + q^4 + 2*q^7 + 4*q^10 + 6*q^13 + 9*q^16 + 14*q^19 + 20*q^22 + 29*q^25 + ...
MATHEMATICA
QP = QPochhammer; s=QP[q^6]^2/(QP[q]*QP[q^3]) + O[q]^50; CoefficientList[s, q] (* Jean-François Alcover, Nov 14 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^6 + A)^2 / (eta(x + A) * eta(x^3 + A)), n))}; /* Michael Somos, Aug 19 2006 */
CROSSREFS
Cf. A139135.
Sequence in context: A153140 A295341 A139135 * A260600 A119737 A038718
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Sep 17 2004
EXTENSIONS
Edited (at the suggestion of R. J. Mathar) by N. J. A. Sloane, May 15 2008
STATUS
approved