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A030205 Expansion of q^(-1/2) * eta(q)^2 * eta(q^5)^2 in power of q. 8
1, -2, -1, 2, 1, 0, 2, 2, -6, -4, -4, 6, 1, 4, 6, -4, 0, -2, 2, -4, 6, -10, -1, -6, -3, 12, -6, 0, 8, 12, 2, 2, -2, 2, -12, -12, 2, -2, 0, 8, -11, 6, 6, -12, -6, 4, 8, 4, 2, 0, 6, 14, 4, -6, 2, -4, -6, -6, 2, -12, -11, -12, -1, 2, 20, 0, -8, -4, 18, -4, 12, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

This eta-quotient of conductor 20 is one of the twelve weight 2 newforms listed by Martin and Ono.

The associated elliptic curve is "20a1": y^2 = x^3 + x^2 + 4*x + 4 or "20a2": y^2 = x^3 + x^2 - x.

Number 39 of the 74 eta-quotients listed in Table I of Martin (1996).

The mentioned eta-quotient is in fact eta^2(2*z) * eta^2(10*z) with q = exp(2*Pi*i*tau) with Im(tau) > 0, I^2 = -1, with the q-expansion coefficients b(n) from the M. Somos Oct Aug 13 2006 formula: b(2*n) = 0 and b(2*n+1) = a(n), for n >= 0. A273163(k) = b(prime(k)), k >= 1. See also the comments on multiplicativity of b(n) (called there c(n)) with b(2^k) = b(2)^k = 0, b(5^k) = b(5)^k = (-1)^k, and b(prime(n)^k) = (sqrt(prime(n)))^k*S(k,A273163(n)/sqrt(prime(n))) with Chebyshev's S polynomials (A049310), for n = 2, and n >= 4 and k >= 2. Compare this with the b(p^(e+2)) recurrence given by M. Somos, Oct 31 2005. - Wolfdieter Lang, May 23 2016

LINKS

Seiichi Manyama, Table of n, a(n) for n = 0..1000

M. Koike, On McKay's conjecture, Nagoya Math. J., 95 (1984), 85-89.

LMFDB, Newforms of weight 2 on Gamma_0(20).

Y. Martin, Multiplicative eta-quotients, Trans. Amer. Math. Soc. 348 (1996), no. 12, 4825-4856, see page 4852 Table I.

Y. Martin and K. Ono, Eta-quotients and elliptic curves, Proc. Amer. Math. Soc. 125 (1997), no. 11, 3169-3176. MR1401749 (97m:11057)

K. Ono, Ramanujan, taxicabs, birthdates, ZIP codes and twists, Amer. Math. Monthly, 104 (1997), 912-917. MR1490909 (98i:11020)

Michael Somos, Index to Yves Martin's list of 74 multiplicative eta-quotients and their A-numbers

FORMULA

Euler transform of period 5 sequence [ -2, -2, -2, -2, -4, ...]. - Michael Somos, Oct 31 2005

Given g.f. A(x), then B(x) = q * A(q)^2 satisfies 0 = f(B(q), B(q^2), B(q^4)) where f(u, v, w) = u*w * (u + 8*v + 16*w) - v^3. - Michael Somos, Oct 31 2005

a(n) = b(2*n + 1) where b() is multiplicative with b(2^e) = 0^e, b(5^e) = (-1)^e, else b(p^(e+2)) = b(p)*b(p^(e+1)) - p*b(p^e). - Michael Somos, Oct 31 2005

a(n) = b(2*n + 1) and b(p) = p minus number of points of elliptic curve "20a1" or "20a2" modulo p. - Michael Somos, Aug 13 2006

G.f.: (Product_{k>0} (1 - x^k) * (1 - x^(5*k)))^2.

a(121*n + 60) = -11 * a(n).

Convolution square is A030210. - Michael Somos, Jun 13 2014

a(n) = (-1)^n * A159817(n). - Michael Somos, Jun 10 2015

G.f. is a period 1 Fourier series which satisfies f(-1 / (20 t)) = 20 (t/i)^2 f(t) where q = exp(2 Pi i t). - Michael Somos, Jun 10 2015

EXAMPLE

G.f. = 1 - 2*x - x^2 + 2*x^3 + x^4 + 2*x^6 + 2*x^7 - 6*x^8 - 4*x^9 - 4*x^10 + ...

G.f. of b(n) from eta^2(2*z)*eta^2(10*z) = q - 2*q^3 - q^5 + 2*q^7 + q^9 + 2*q^13 + 2*q^15 - 6*q^17 - 4*q^19 + ..., where q = exp(2*Pi*I*z).

MATHEMATICA

a[ n_] := SeriesCoefficient[ (QPochhammer[ x] QPochhammer[ x^5])^2, {x, 0, n}]; (* Michael Somos, May 28 2013 *)

PROG

(PARI) {a(n) = if( n<0, 0, n = 2*n + 1; ellan( ellinit( [0, 1, 0, 4, 4], 1), n)[n])}; /* Michael Somos, Oct 31 2005 */

(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( (eta(x + A) * eta(x^5 + A))^2, n))}; /* Michael Somos, Oct 31 2005 */

(PARI) {a(n) = if( n<0, 0, n = 2*n + 1; ellan( ellinit( [0, 1, 0, -1, 0], 1), n)[n])}; /* Michael Somos, Aug 13 2006 */

(PARI) {a(n) = my(A, p, e, x, y, a0, a1); if( n<0, 0, n = 2*n + 1; A = factor(n); prod( k=1, matsize(A)[1], [p, e] = A[k, ]; if( p==2, 0, p==5, (-1)^e, a0=1; a1 = y = -sum( x=0, p-1, kronecker( x^3 + x^2 - x, p)); for( i=2, e, x = y*a1 - p*a0; a0=a1; a1=x); a1)))}; /* Michael Somos, Aug 13 2006 */

(Sage) CuspForms( Gamma0(20), 2, prec=92).0; # Michael Somos, May 28 2013

(MAGMA) Basis( CuspForms( Gamma0(20), 2), 145) [1]; /* Michael Somos, May 27 2014 */

(MAGMA) A := Basis( CuspForms( Gamma1(20), 2), 145); A[1] - 2*A[3]; /* Michael Somos, May 17 2015 */

CROSSREFS

Cf. A030210, A049310, A159817, A273163.

Sequence in context: A122807 A105700 A236831 * A159817 A079532 A328176

Adjacent sequences:  A030202 A030203 A030204 * A030206 A030207 A030208

KEYWORD

sign

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified October 18 18:56 EDT 2019. Contains 328197 sequences. (Running on oeis4.)