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 A243747 Expansion of (phi(q) - phi(q^2))^2 / 4 in powers of q where phi() is a Ramanujan theta function. 2
 1, -2, 1, 2, -2, 0, 1, -2, 4, -2, -2, 2, 0, 0, 1, 0, -1, -2, 4, 0, -2, 0, -2, 2, 4, -4, 0, 2, 0, 0, 1, -4, 2, 0, -1, 2, -2, 0, 4, 0, 0, -2, -2, 2, 0, 0, -2, 0, 5, -4, 4, 2, -4, 0, 0, -4, 4, -2, 0, 2, 0, 0, 1, 4, -4, -2, 2, 0, 0, 0, -1, 0, 4, -2, -2, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700). Expansion of third basis element of modular forms space for Gamma_1(8) of weight 1 in powers of q. LINKS G. C. Greubel, Table of n, a(n) for n = 2..2500 Michael Somos, Introduction to Ramanujan theta functions Eric Weisstein's World of Mathematics, Ramanujan Theta Functions FORMULA Expansion of (q * f(-q, -q^7)^2 / psi(-q))^2 in powers of q where psi(), f() are Ramanujan theta functions. Euler transform of period 8 sequence [-2, 0, 2, 2, 2, 0, -2, -2, ...]. G.f.: (theta_3(x) - theta_3(x^2))^2 / 4 = (Sum_{k>0} x^(k^2) - x^(2k^2))^2. Convolution square of A143259. EXAMPLE G.f. = q^2 - 2*q^3 + q^4 + 2*q^5 - 2*q^6 + q^8 - 2*q^9 + 4*q^10 - 2*q^11 + ... MATHEMATICA a[ n_] := SeriesCoefficient[ (EllipticTheta[ 3, 0, q] - EllipticTheta[ 3, 0, q^2])^2 / 4, {q, 0, n}]; PROG (PARI) {a(n) = if( n<2, 0, sum(k=1, n-1, (issquare(k) - issquare(2*k)) * (issquare(n - k) - issquare(2*n - 2*k))))}; (Sage) ModularForms( Gamma1(8), 1, prec=70).2 (MAGMA) Basis( ModularForms( Gamma1(8), 1), 70) [3]; CROSSREFS CF. A143259. Sequence in context: A071858 A122864 A140084 * A105937 A035146 A035216 Adjacent sequences:  A243744 A243745 A243746 * A243748 A243749 A243750 KEYWORD sign AUTHOR Michael Somos, Jun 09 2014 STATUS approved

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Last modified April 22 14:33 EDT 2021. Contains 343177 sequences. (Running on oeis4.)