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A033719
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Coefficients in expansion of theta_3(q) * theta_3(q^7) in powers of q.
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7
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1, 2, 0, 0, 2, 0, 0, 2, 4, 2, 0, 4, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 4, 0, 2, 0, 0, 2, 4, 0, 0, 8, 0, 0, 0, 2, 4, 0, 0, 0, 0, 0, 4, 4, 0, 0, 0, 0, 2, 0, 0, 0, 4, 0, 0, 4, 0, 0, 0, 0, 0, 0, 2, 10, 0, 0, 4, 0, 0, 0, 4, 4, 0, 0, 0, 0, 4, 0, 4, 0, 2, 0, 0, 0, 0, 0, 0, 8, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 4, 2, 0, 0, 0
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OFFSET
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0,2
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COMMENTS
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Number of integer solutions to the equation x^2 + 7*y^2 = n.
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REFERENCES
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J. H. Conway and N. J. A. Sloane, "Sphere Packings, Lattices and Groups", Springer-Verlag, p 102 eq 9.
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LINKS
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FORMULA
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Coefficients in expansion of Sum_{ i, j = -inf .. inf } q^(i^2 + 7*j^2).
Euler transform of period 28 sequence [ 2, -3, 2, -1, 2, -3, 4, -1, 2, -3, 2, -1, 2, -6, 2, -1, 2, -3, 2, -1, 4, -3, 2, -1, 2, -3, 2, -2, ...].
Expansion of (eta(q^2) * eta(q^14))^5 / (eta(q) * eta(q^4) * eta(q^7) * eta(q^28))^2 in powers of q.
G.f.: Product_{k>0} (1 - x^(2*k)) * (1 + x^(2*k-1))^2 * (1 - x^(14*k)) * (1 + x^(14*k-7))^2.
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EXAMPLE
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G.f. = 1 + 2*x + 2*x^4 + 2*x67 + 4*x^8 + 2*x^9 + 4*x^11 + 6*x^16 + 4*x^23 + ...
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MAPLE
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seq(coeff(series(mul((1-x^(2*k))*(1+x^(2*k-1))^2*(1-x^(14*k))*(1+x^(14*k-7))^2, k=1..n), x, n+1), x, n), n = 0 .. 110); # Muniru A Asiru, Feb 02 2019
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MATHEMATICA
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a[ n_] := SeriesCoefficient[ EllipticTheta[ 3, 0, x] EllipticTheta[ 3, 0, x^7], {x, 0, n}]; (* Michael Somos, Feb 22 2015 *)
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PROG
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(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^-2 * eta(x^2 + A)^5 * eta(x^4 + A)^-2 * eta(x^7 + A)^-2 * eta(x^14 + A)^5 * eta(x^28 + A)^-2, n))};
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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