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A160806
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Expansion of q^(-1/3) * (eta(q) * eta(q^7) + eta(q^4) * eta(q^28)) in powers of q^2.
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1, -1, 0, 0, 1, 0, -2, -2, 1, 0, 0, 2, 0, 2, 0, 0, 0, 0, -2, 0, -1, 2, 0, 0, 0, -2, 0, 2, 1, -1, 0, 0, -2, 0, 0, -2, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 2, 0, 1, 0, 2, 0, 0, 0, 0, -2, -2, -1, 0, 0, 1, 0, 2, -2, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, -2, 2, -2, 0, 0, -2, 0, -2, 0, 0, 0, 0, -1, 0, 2, 2, -2, 0, 0, 2, 0, 0, 0, 0, 0, 0, 2, 0, 1
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,7
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FORMULA
| a(n) = b(6*n + 1) where b(n) is multiplicative and b(2^e) = b(3^e) = 0^e, b(p^e) = (-1)^e if p = 7, b(p^e) = (1+(-1)^e)/2 if p == 3, 5, 6 (mod 7), else p == 1, 2, 4 (mod 7) and p=y^2+7x^2 when b(p^2e) = (-1)^e if x*y not divisible by 3, b(p^e) = e+1 if x divisible by 3 or (e+1)(-1)^e if y divisible by 3.
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EXAMPLE
| q - q^7 + q^25 - 2*q^37 - 2*q^43 + q^49 + 2*q^67 + 2*q^79 - 2*q^109 + ...
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PROG
| (PARI) {a(n) = local(A, p, e, x, y); if(n<0, 0, n = 6*n + 1; A = factor(n); prod(k=1, matsize(A)[1], if(p=A[k, 1], e=A[k, 2]; if(p==7, (-1)^e, if(kronecker(p, 7)==-1, !(e%2), for(x=0, sqrtint(p\7), if(issquare(p - 7*x^2, &y), y=if(x%3&y%3, real(I^e), (e+1) * if(x%3, (-1)^e, 1)); break)); y)))))}
(PARI) {a(n) = local(A); if(n<0, 0, n *= 2; A = x * O(x^n); polcoeff( eta(x + A) * eta(x^7 + A), n))}
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CROSSREFS
| A002655(2*n) = a(n).
Sequence in context: A015199 A051168 A163528 * A191411 A133418 A181169
Adjacent sequences: A160803 A160804 A160805 * A160807 A160808 A160809
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KEYWORD
| sign
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AUTHOR
| Michael Somos, May 26 2009
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