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 A321528 Expansion of b(x)^2 * b(x^2) / b(x^4) where b is a cubic AGM theta function. 1
 1, -6, 6, 30, -66, -36, 186, -48, -210, 138, 36, -72, 114, -84, 48, 180, -498, -108, 726, -120, -396, 240, 72, -144, -30, -186, 84, 462, -528, -180, 1116, -192, -1074, 360, 108, -288, 654, -228, 120, 420, -1260, -252, 1488, -264, -792, 828, 144, -288, -318 (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). Cubic AGM theta functions: a(q) (see A004016), b(q) (A005928), c(q) (A005882). Number 64 of the 126 eta-quotients listed in Table 1 of Williams 2012. LINKS Michael Somos, Introduction to Ramanujan theta functions Eric Weisstein's World of Mathematics, Ramanujan Theta Functions K. S. Williams, Fourier series of a class of eta quotients, Int. J. Number Theory 8 (2012), no. 4, 993-1004. FORMULA Expansion of phi(-x) * phi(-x^2)^3 / (phi(-x^3) * phi(-x^6)) in powers of x where phi() is a Ramanujan theta function. Expansion of eta(q)^6 * eta(q^2)^3 * eta(q^12) / (eta(q^3)^2 * eta(q^4)^3 * eta(q^6)) in powers of q. G.f. is a period 1 Fourier series which satisfies f(-1 / (12 t)) = 864 (t / i)^2 g(t) where q = exp(2 Pi i t) and g() is the g.f. for A321527. a(n) = -6 * (s(n/1) - 4*s(n/2) - 9*s(n/3) + 16*s(n/4)) if n>0, where s(x) = sum of divisors of x for integer x else 0. a(2*n + 1) = -6 * A134077(n). a(6*n + 5) = -a(12*n + 10) = -36 * A098098(n). EXAMPLE G.f. = 1 - 6*x + 6*x^2 + 30*x^3 - 66*x^4 - 36*x^5 + 186*x^6 - 48*x^7 + ... MATHEMATICA a[ n_] := SeriesCoefficient[ (EllipticTheta[ 4, 0, x] EllipticTheta[ 4, 0, x^2])^3 / ( EllipticTheta[ 4, 0, x^3] EllipticTheta[ 4, 0, x^6]), {x, 0, n}]; a[ n_] := With[ {s = If[ FractionalPart @ # > 0, 0, DivisorSigma[1, #]] &}, If[ n < 1, Boole[n==0], -6 (s[n/1] - 4 s[n/2] - 9 s[n/3] + 16 s[n/4])]]; a[ n_] := If[ n < 1, Boole[n==0], -6 Sum[ d {1, -1, -2, 3, 1, -4, 1, 3, -2, -1, 1, 0}[[Mod[d, 12, 1]]], {d, Divisors[n]}]]; PROG (PARI) {a(n) = if( n<1, n==0, -6 * sumdiv( n, d, d * [0, 1, -1, -2, 3, 1, -4, 1, 3, -2, -1, 1][d%12 + 1]))}; (PARI) {a(n) = my(s = x -> if(frac(x), 0, sigma(x))); if( n<1, n==0, -6 * (s(n/1) - 4*s(n/2) - 9*s(n/3) + 16*s(n/4)))}; (PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^6 * eta(x^2 + A)^3 * eta(x^12 + A) / (eta(x^3 + A)^2 * eta(x^4 + A)^3 * eta(x^6 + A)), n))}; (Magma) A := Basis( ModularForms( Gamma0(12), 2), 49); A[1] - 6*A[2] + 6*A[3] + 30*A[4] - 66*A[5]; CROSSREFS Cf. A098098, A134077, A321527. Sequence in context: A147799 A071021 A279865 * A074002 A359741 A140959 Adjacent sequences: A321525 A321526 A321527 * A321529 A321530 A321531 KEYWORD sign AUTHOR Michael Somos, Nov 12 2018 STATUS approved

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Last modified March 30 07:09 EDT 2023. Contains 361606 sequences. (Running on oeis4.)