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A260413
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Expansion of chi(-x) where chi() is a 3rd order mock theta function.
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1
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1, -1, 1, 0, 0, 0, 1, -1, 0, 0, -1, 0, 1, -1, 1, 1, 0, 0, 0, -1, 0, 0, -1, 0, 1, -1, 1, 0, -1, 1, 1, -1, 0, 1, -1, 0, 1, -2, 1, 1, -1, 0, 1, -1, 0, 1, -2, 0, 1, -2, 1, 1, -1, 1, 1, -2, 1, 1, -2, 1, 2, -2, 1, 1, -2, 1, 1, -2, 0, 1, -3, 0, 2, -3, 2, 2, -2, 1, 2
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OFFSET
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0,38
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LINKS
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FORMULA
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G.f.: Sum_{k>=0} (-x)^k^2 / ((1 - x + x^2) * (1 + x^2 + x^4) ... (1 - (-x)^k + x^(2*k))).
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EXAMPLE
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G.f. = 1 - x + x^2 + x^6 - x^7 - x^10 + x^12 - x^13 + x^14 + x^15 - x^19 + ...
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MATHEMATICA
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a[ n_] := If[ n < 0, 0, SeriesCoefficient[ Sum[ (-x)^k^2 / Product[ 1 - (-x)^i + x^(2 i), {i, k}], {k, 0, Sqrt @ n}], {x, 0, n}]];
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PROG
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(PARI) {a(n) = if( n<0, 0, polcoeff( sum(k=0, sqrtint(n), (-x)^k^2 / prod(i=1, k, 1 - (-x)^i + x^(2*i), 1 + x * O(x^(n - k^2)))), n))};
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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STATUS
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approved
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