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 A138949 Expansion of (3 * phi(q^3)^2 - phi(q)^2) / 2 in powers of q where phi() is a Ramanujan theta function. 7
 1, -2, -2, 6, -2, -4, 6, 0, -2, -2, -4, 0, 6, -4, 0, 12, -2, -4, -2, 0, -4, 0, 0, 0, 6, -6, -4, 6, 0, -4, 12, 0, -2, 0, -4, 0, -2, -4, 0, 12, -4, -4, 0, 0, 0, -4, 0, 0, 6, -2, -6, 12, -4, -4, 6, 0, 0, 0, -4, 0, 12, -4, 0, 0, -2, -8, 0, 0, -4, 0, 0, 0, -2, -4 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700). LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 L.-C. Shen, On the Modular Equations of Degree 3, Proc. Amer. Math. Soc. 122 (1994), no. 4, 1101-1114. See p. 1108, Eq. (3.20). Eric Weisstein's World of Mathematics, Ramanujan Theta Functions FORMULA Expansion of phi(-q) * phi(-q^2) * chi(q^3) / chi(-q^3) in powers of q where phi(), chi() are Ramanujan theta functions. Expansion of eta(q)^2 * eta(q^2) * eta(q^6)^3 / (eta(q^3)^2 * eta(q^4) * eta(q^12)) in powers of q. Euler transform of period 12 sequence [ -2, -3, 0, -2, -2, -4, -2, -2, 0, -3, -2, -2, ...]. Moebius transform is period 12 sequence [ -2, 0, 8, 0, -2, 0, 2, 0, -8, 0, 2, 0, ...]. a(n) = -2 * b(n) where b() is multiplicative and b(2^e) = 1, b(3^e) = -1 + 2 * (-1)^e, b(p^e) = e+1 if p == 1, 5 (mod 12), b(p^e) = (1 + (-1)^e) / 2 if p == 7, 11 (mod 12). G.f. is a period 1 Fourier series which satisfies f(-1 / (12 t)) = 12 (t/i) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A113446. G.f.: Product_{k>0} (1 - x^(2*k))^2 * (1 - x^k + x^(2*k))^2 / ((1 + x^(2*k))^2 * (1 - x^(2*k) + x^(4*k))). G.f.: 1 - 2 * Sum_{k>0} (f(3*k - 2) + f(3*k - 1) - 2 * f(3*k)) where f(n) := x^n / (1 + x^(2*n)). a(12*n + 7) = a(12*n + 11) = 0. a(2*n) = a(n). a(n) = -2 * A138950(n) unless n=0. a(2*n + 1) = -2 * A116604(n). a(3*n + 1) = A122865(n). a(3*n + 2) = -2 * A122856(n). a(4*n + 1) = -2 * A008441(n). EXAMPLE G.f. = 1 - 2*q - 2*q^2 + 6*q^3 - 2*q^4 - 4*q^5 + 6*q^6 - 2*q^8 - 2*q^9 + ... MATHEMATICA a[ n_] := SeriesCoefficient[ (3 EllipticTheta[ 3, 0, q^3]^2 - EllipticTheta[ 3, 0, q]^2) / 2, {q, 0, n}]; (* Michael Somos, Sep 07 2015 *) a[ n_] := If[ n < 1, Boole[n == 0], -2 DivisorSum[ n, KroneckerSymbol[ -4, n/#] {1, 1, -2}[[Mod[#, 3, 1]]] &]]; (* Michael Somos, Sep 07 2015 *) PROG (PARI) {a(n) = if( n<1, n==0, 2 * sumdiv(n, d, kronecker(-4, n/d) * [2, -1, -1][d%3 + 1]))}; (PARI) {a(n) = my(A, p, e); if( n<1, n==0, A = factor(n); -2 * prod( k=1, matsize(A)[1], [p, e] = A[k, ]; if( p==2, 1, p==3, -1 + 2 * (-1)^e, p%12 < 6, e+1, 1-e%2))) }; (PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^2 * eta(x^2 + A) * eta(x^6 + A)^3 / (eta(x^3 + A)^2 * eta(x^4 + A) * eta(x^12 + A)), n))}; CROSSREFS Cf. A008441, A116604, A122856, A122865, A138950. Sequence in context: A021446 A062401 A286383 * A138951 A163370 A278159 Adjacent sequences:  A138946 A138947 A138948 * A138950 A138951 A138952 KEYWORD sign AUTHOR Michael Somos, Apr 03 2008 STATUS approved

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Last modified February 18 21:20 EST 2020. Contains 332028 sequences. (Running on oeis4.)