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A107505
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Theta series of quadratic form with Gram matrix [ 2, 1, 0, 1; 1, 4, 1, 0; 0, 1, 4, -2; 1, 0, -2, 8].
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9
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1, 2, 6, 8, 14, 12, 24, 16, 30, 26, 36, 24, 56, 2, 48, 48, 62, 36, 78, 40, 84, 64, 72, 48, 120, 62, 6, 80, 112, 60, 144, 64, 126, 96, 108, 96, 182, 76, 120, 8, 180, 84, 192, 88, 168, 156, 144, 96, 248, 114, 186, 144, 14, 108, 240, 144, 240, 160, 180, 120
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OFFSET
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0,2
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COMMENTS
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Coefficients of a theta series associated with a certain "Haupt-form" of rank 4 and level 13.
The Gram matrix is denoted by A in Parry 1979 on page 165.
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LINKS
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FORMULA
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a(n) = 2 * b(n) where b() is multiplicative and b(13^e) = 1, b(p^e) = (p^(e+1) - 1) / (p - 1) otherwise. - Michael Somos, Mar 23 2012
G.f. is a period 1 Fourier series which satisfies f(-1 / (13 t)) = 13 (t/i)^2 f(t) where q = exp(2 Pi i t). - Michael Somos, Mar 23 2012
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EXAMPLE
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G.f. = 1 + 2*q + 6*q^2 + 8*q^3 + 14*q^4 + 12*q^5 + 24*q^6 + 16*q^7 + 30*q^8 + ...
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MATHEMATICA
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a[n_] := If[n == 0, 1, 2 DivisorSigma[1, n/13^IntegerExponent[n, 13]]];
a[n_] := If[n == 0, 1, 2 DivisorSum[n, Boole[!Divisible[#, 13]] # &]];
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PROG
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(PARI) {a(n) = if( n<1, n==0, 1, 2 * sigma(n / 13^valuation(n, 13)))}; /* Michael Somos, Mar 23 2012 */
(PARI) {a(n) = my(G); if( n<0, 0, G = [2, 1, 0, 1; 1, 4, 1, 0; 0, 1, 4, -2; 1, 0, -2, 8]; polcoeff( 1 + 2 * x * Ser(qfrep( G, n, 1)), n))}; /* Michael Somos, Mar 23 2012 */
(Sage) ModularForms( Gamma0(13), 2, prec=100).0; # Michael Somos, Jun 27 2013
(Magma) Basis( ModularForms( Gamma0(13), 2), 100) [1]; /* Michael Somos, Aug 15 2016 */
(Magma) [Coefficient(Basis(ModularForms(Gamma0(13), 2))[1], n) : n in [0..100] ]; // Vincenzo Librandi, Jun 27 2017
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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