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 A318734 a(n) = Sum_{k=1..n} (-1)^(k + 1) * d(2*k - 1), where d(k) is the number of divisors of k (A000005). 5
 1, -1, 1, -1, 2, 0, 2, -2, 0, -2, 2, 0, 3, -1, 1, -1, 3, -1, 1, -3, -1, -3, 3, 1, 4, 0, 2, -2, 2, 0, 2, -4, 0, -2, 2, 0, 2, -4, 0, -2, 3, 1, 5, 1, 3, -1, 3, -1, 1, -5, -3, -5, 3, 1, 3, -1, 1, -3, 3, -1, 2, -2, 2, 0, 4, 2, 6, -2, 0, -2, 2, -2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 LINKS Hugo Pfoertner, Table of n, a(n) for n = 1..10000 Hugo Pfoertner, Records of A318734, showing negative and positive records. MATHEMATICA a[n_] := Sum[(-1)^(k + 1) DivisorSigma[0, 2 k - 1], {k, 1, n}]; Array[a, 100] (* Jean-François Alcover, Sep 17 2018 *) PROG (PARI) s=0; j=-1; forstep(k=1, 141, 2, j=-j; s=s+j*numdiv(k); print1(s, ", ")) (PARI) a(n) = sum(k=1, n, (-1)^(k+1)*numdiv(2*k-1)); \\ Michel Marcus, Sep 08 2018 CROSSREFS Cf. A000005, A099774, A006218, A222068. Records and their positions: A318735, A318736, A318737, A318738. Sequence in context: A123063 A031358 A279103 * A029317 A127800 A035692 Adjacent sequences:  A318731 A318732 A318733 * A318735 A318736 A318737 KEYWORD sign,look AUTHOR Hugo Pfoertner, Sep 05 2018 STATUS approved

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Last modified June 17 16:56 EDT 2019. Contains 324196 sequences. (Running on oeis4.)