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A263828
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The number c_{P c pi_1(B_1)}(n) of the first amphicosm n-coverings over the first amphicosm.
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2
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1, 4, 5, 10, 7, 20, 9, 22, 18, 28, 13, 50, 15, 36, 35, 46, 19, 72, 21, 70, 45, 52, 25, 110, 38, 60, 58, 90, 31, 140, 33, 94, 65, 76, 63, 180, 39, 84, 75, 154, 43, 180, 45, 130, 126, 100, 49, 230, 66, 152, 95, 150, 55, 232, 91, 198, 105, 124, 61
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OFFSET
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1,2
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LINKS
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MATHEMATICA
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a[n_] := Sum[(3/2 + 1/2 (-1)^Mod[d, 2]) DivisorSigma[1, n/d], {d, Divisors[ n]}] - If[OddQ[n], 0, Sum[(3/2 + 1/2 (-1)^Mod[d, 2]) DivisorSigma[1, n/(2 d)], {d, Divisors[n/2]}]];
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PROG
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(PARI)
a(n) = {
sumdiv(n, d, (3/2 + 1/2*(-1)^(d%2)) * sigma(n/d)) -
if (n%2, 0, sumdiv(n\2, d, (3/2 + 1/2*(-1)^(d%2))*sigma(n\(2*d))))
};
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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