OFFSET
1,2
COMMENTS
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
Sum_{k=1..n} a(k) ~ c * n^3, where c = (16-10*sqrt(2))*zeta(3)/Pi^2 = 0.226276... . - Amiram Eldar, Nov 28 2023
MATHEMATICA
Select[DivisorSigma[1, Range[1000]], OddQ[#] &] (* Giovanni Resta, Jan 08 2020 *)
With[{max = 1000}, DivisorSigma[1, Union[Range[Sqrt[max]]^2, 2*Range[Sqrt[max/2]]^2]]] (* Amiram Eldar, Nov 28 2023 *)
PROG
(PARI) A152677_upto(lim)=apply(sigma, vecsort(concat(vector(sqrtint(lim\1), i, i^2), vector(sqrtint(lim\2), i, 2*i^2)))) \\ Gives [a(n) = sigma(k) with k = A028982(n) <= lim]. - Charles R Greathouse IV, Feb 15 2013, corrected by M. F. Hasler, Jan 08 2020
(Magma) [d:k in [1..1000]|IsOdd(d) where d is DivisorSigma(1, k)]; // Marius A. Burtea, Jan 09 2020
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Omar E. Pol, Dec 10 2008
EXTENSIONS
Extended by R. J. Mathar, Dec 12 2008
Edited and definition reworded by M. F. Hasler, Jan 08 2020
STATUS
approved