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 A053807 a(n) = Sum_{k=1..n, n mod k = 1} k^2. 5
 0, 0, 4, 9, 20, 25, 49, 49, 84, 90, 129, 121, 209, 169, 249, 259, 340, 289, 454, 361, 545, 499, 609, 529, 849, 650, 849, 819, 1049, 841, 1299, 961, 1364, 1219, 1449, 1299, 1910, 1369, 1809, 1699, 2209, 1681, 2499, 1849, 2561, 2365, 2649 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 FORMULA a(n) = sigma_2(n-1) - 1 for n >= 2. G.f.: -x^2/(1 - x) + Sum_{k>=1} k^2*x^(k+1)/(1 - x^k). - Ilya Gutkovskiy, Mar 17 2017 MAPLE f:= n -> numtheory:-sigma[2](n-1) - 1: f(1):= 0: map(f, [\$1..100]); # Robert Israel, Jan 29 2018 MATHEMATICA a[1] = 0; a[n_] := DivisorSigma[2, n - 1] - 1; Array[a, 50] (* Jean-François Alcover, Feb 28 2019 *) PROG (PARI) concat([0], vector(50, n, n++; sigma(n-1, 2) -1)) \\ G. C. Greubel, Feb 28 2019 (MAGMA) [0] cat [DivisorSigma(2, n-1) -1: n in [2..50]]; // G. C. Greubel, Feb 28 2019 (Sage) [0] + [sigma(n-1, 2) - 1 for n in (2..50)] # G. C. Greubel, Feb 28 2019 CROSSREFS Cf. A001157. Sequence in context: A288100 A063454 A288104 * A327238 A066109 A030734 Adjacent sequences:  A053804 A053805 A053806 * A053808 A053809 A053810 KEYWORD nonn,look AUTHOR N. J. A. Sloane, Apr 07 2000 STATUS approved

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Last modified September 16 08:36 EDT 2019. Contains 327091 sequences. (Running on oeis4.)