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
KEYWORD
nonn,look
AUTHOR
N. J. A. Sloane, Apr 07 2000
STATUS
approved