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

1,2

Comments

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

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

Links

Giovanni Resta, Table of n, a(n) for n = 1..10000

Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).

Example

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

Mathematica

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

Prog

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

Crossrefs

Cf. A000203, A002191, A007369.

Sequence in context: A002254 A356211 A331036 * A192148 A151875 A117533

Adjacent sequences: A060654 A060655 A060656 * A060658 A060659 A060660

Keyword

nonn

Author

Robert G. Wilson v, Apr 18 2001

Extensions

Name edited by Giovanni Resta, Jan 08 2020

Status

approved