A072461 Numbers k such that sigma(k) == 1 (mod 4).

1, 9, 49, 50, 81, 100, 121, 200, 338, 361, 400, 441, 450, 529, 578, 625, 676, 729, 800, 900, 961, 1089, 1156, 1352, 1600, 1682, 1800, 1849, 2209, 2312, 2401, 2450, 2704, 2738, 3042, 3200, 3249, 3362, 3364, 3481, 3600, 3969, 4050, 4225, 4489, 4624, 4761

Offset: 1

Author

Labos Elemer, Jun 19 2002

Keywords

• nonn

Links

• Ray Chandler, Table of n, a(n) for n = 1..10000 (first 1000 terms from Harvey P. Dale)

Crossrefs

Cf. A000203, A028982, A072462, A081339.

Positions of 1's in A105824.

Programs

Mathematica

Select[Range[5000],Mod[DivisorSigma[1,#],4]==1&] (* Harvey P. Dale, Oct 24 2016 *)

PARI

isok(n) = ((sigma(n) % 4) == 1);

Extensions

Name edited by Michel Marcus, Dec 19 2013 and Amiram Eldar, Jun 26 2024