A060657 Odd values of the sum-of-divisors function sigma (A000203) (without repetitions).

1, 3, 7, 13, 15, 31, 39, 57, 63, 91, 93, 121, 127, 133, 171, 183, 195, 217, 255, 307, 363, 381, 399, 403, 465, 511, 549, 553, 741, 781, 819, 847, 855, 871, 921, 931, 961, 993, 1023, 1093, 1143, 1209, 1281, 1407, 1651, 1659, 1723, 1729, 1767, 1815, 1893, 1953

Offset: 1

Author

Robert G. Wilson v, Apr 18 2001

Keywords

• nonn

Comments

That is, the odd values produced by the sigma function.

Odd terms of A002191. - Michel Marcus, Jun 10 2014

Examples

			a(7) = 39 because sigma(18) = 1+2+3+6+9+18 = 39, an odd number.
		

Links

• Giovanni Resta, Table of n, a(n) for n = 1..10000
• Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Crossrefs

Cf. A000203, A002191, A007369.

Programs

Mathematica

nn = 2000; Union[Select[DivisorSigma[1, Range[nn]], OddQ[#] && # <= nn &]]  (* Harvey P. Dale, Mar 12 2011 *)

PARI

is(k) = k % 2 && invsigmaNum(k) > 0; \\ Amiram Eldar, Dec 26 2024, using Max Alekseyev's invphi.gp

Extensions

Name edited by Giovanni Resta, Jan 08 2020