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Search: id:A002656
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| A002656 |
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Expansion of q * Product ((1-q^k)(1-q^{7k}))^3, k=1..inf. |
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+0
4
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| 1, -3, 0, 5, 0, 0, -7, -3, 9, 0, -6, 0, 0, 21, 0, -11, 0, -27, 0, 0, 0, 18, 18, 0, 25, 0, 0, -35, -54, 0, 0, 45, 0, 0, 0, 45, -38, 0, 0, 0, 0, 0, 58, -30, 0, -54, 0, 0, 49, -75, 0, 0, -6, 0, 0, 21, 0, 162, 0, 0, 0, 0, -63, -91, 0, 0, -118, 0, 0, 0, 114, -27, 0, 114, 0, 0, 42, 0, -94, 0, 81, 0, 0, 0, 0, -174, 0
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Expansion of (eta(q) * eta(q^7))^3 in powers of q.
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REFERENCES
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B. Berndt, Commentary on Ramanujan's Papers, pp. 357-426 of Collected Papers of Srinivasa Ramanujan, Ed. G. H. Hardy et al., AMS Chelsea 2000. See page 372 (4).
N. Elkies, The Klein quartic in number theory, pp. 51-101 of S. Levy, ed., The Eightfold Way, Cambridge Univ. Press, 1999. MR1722413 (2001a:11103)
N. Koblitz, Introduction to Elliptic Curves and Modular Forms, Springer-Verlag, 1984, see p. 145, problem 13.
F. Garvan, D. Kim and D. Stanton, Cranks and t-cores, Invent. Math. 101 (1990), no. 1, 1-17. see pp 9-10.
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LINKS
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N. Elkies, The Klein quartic in number theory
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FORMULA
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G.f.: (1/2)* Sum_{u,v} (u*u -2*v*v)* x^(u*u +u*v +2*v*v). - Michael Somos Jun 14 2007
Euler transform of period 7 sequence [ -3, -3, -3, -3, -3, -3, -6, ...]. - Michael Somos Mar 11 2004
a(n) is multiplicative and a(7^e) = (-7)^e, a(p^e) = p^e * (1 + (-1)^e) / 2 if p == 3, 5, 6 (mod 7), a(p^e) = a(p) * a(p^(e-1)) - p^2 * a(p^(e-2)) and a(2) = -3, a(p) = 2 * (x^2 - 7*y^2) where p = x^2 + 7*y^2 if p == 1, 2, 4 (mod 7). - Michael Somos Apr 12 2008.
G.f. A(x) satisfies 0 = f(A(x), A(x^2), A(x^4)) where f(u, v, w) = u * w * (u + 6*v + 8*w) - v^3. - Michael Somos May 02 2005.
G.f. is a period 1 Fourier series which satisfies f(-1 / (7 t)) = 7^(3/2) (t/i)^3 f(t) where q = exp(2 pi i t). - Michael Somos Apr 12 2008
a(7*n + 3) = a(7*n + 5) = a(7*n + 6) = 0. - Michael Somos Oct 19 2005
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EXAMPLE
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q - 3*q^2 + 5*q^4 - 7*q^7 - 3*q^8 + 9*q^9 - 6*q^11 + 21*q^14 - 11*q^16 + ...
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PROGRAM
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(PARI) {a(n) = local(A); if( n<1, 0, n--; A = x * O(x^n); polcoeff( (eta(x + A) * eta(x^7 + A))^3, n))} /* Michael Somos Apr 16 2005 */
(PARI) {a(n) = local(A, p, e, x, y, a0, a1); if( n<1, 0, A = factor(n); prod(k=1, matsize(A)[1], if(p = A[k, 1], e = A[k, 2]; if( kronecker(-7, p)<1, if( p==7, (-1)^e, 1-e%2) * p^e, for(i=1, sqrtint(p\7), if( issquare(p - 7*i^2), y=i; break)); a0 = 1; a1 = y = if( p==2, -3, 2*p - 28*y^2); for(i=2, e, x = y * a1 - p^2 * a0; a0 = a1; a1 = x); a1))))} /* Michael Somos Oct 19 2005 */
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CROSSREFS
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Sequence in context: A133089 A136599 A131986 this_sequence A166586 A122274 A003966
Adjacent sequences: A002653 A002654 A002655 this_sequence A002657 A002658 A002659
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KEYWORD
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sign,mult
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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