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A112128
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Expansion of phi(q^4) / phi(q) in powers of q where phi() is a Ramanujan theta function.
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5
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1, -2, 4, -8, 16, -28, 48, -80, 128, -202, 312, -472, 704, -1036, 1504, -2160, 3072, -4324, 6036, -8360, 11488, -15680, 21264, -28656, 38400, -51182, 67864, -89552, 117632, -153836, 200352, -259904, 335872, -432480, 554952, -709728, 904784, -1149916, 1457136
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OFFSET
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0,2
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COMMENTS
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LINKS
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FORMULA
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Expansion of (eta(q) / eta(q^16))^2 * (eta(q^8) / eta(q^2))^5 in powers of q.
Euler transform of period 16 sequence [ -2, 3, -2, 3, -2, 3, -2, -2, -2, 3, -2, 3, -2, 3, -2, 0, ...].
G.f. A(x) satisfies 0 = f(A(x), A(x^2)) where f(u, v) = u^2 - (1 - 2*u + 2*u^2) * (1 - 2*v + 2*v^2).
G.f.: (Sum_{k in Z} x^(4*k^2)) / (Sum_{k in Z} x^(k^2)) = theta_3(0, x^4) / theta_3(0, x).
G.f.: Product_{k>0} ((1 + x^(2*k)) * (1 + x^(4*k)))^3 / ((1 + x^k) * (1 + x^(8*k)))^2.
Expansion of continued fraction 1 / (1 + 2*x / (1 - x^2 + (x^1 + x^3)^2 / (1 - x^6 + (x^2 + x^6)^2 / (1 - x^10 + (x^3 + x^9)^2 / ...)))).
G.f. is a period 1 Fourier series which satisfies f(-1 / (16 t)) = 1/2 * g(t) where q = exp(2 Pi i t) and g() is the g.f. for A208724.
a(n) ~ (-1)^n * exp(sqrt(n)*Pi) / (2^(7/2) * n^(3/4)). - Vaclav Kotesovec, Nov 15 2017
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EXAMPLE
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G.f. = 1 - 2*q + 4*q^2 - 8*q^3 + 16*q^4 - 28*q^5 + 48*q^6 - 80*q^7 + 128*q^8 + ...
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MATHEMATICA
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QP = QPochhammer; s = QP[q]^2*(QP[q^8]^5/QP[q^2]^5/QP[q^16]^2) + O[q]^40; CoefficientList[s, q] (* Jean-François Alcover, Nov 30 2015, adapted from PARI *)
a[ n_] := SeriesCoefficient[ EllipticTheta[ 3, 0, q^4] / EllipticTheta[ 3, 0, q], {q, 0, n}]; (* Michael Somos, Dec 11 2016 *)
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PROG
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(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^2 * eta(x^8 + A)^5 / (eta(x^2 + A)^5 * eta(x^16 + A)^2), n))};
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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STATUS
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approved
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