A163162 Numbers k such that sigma(k) is not prime.

1, 3, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 17, 18, 19, 20, 21, 22, 23, 24, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77

Offset

1,2

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Formula

A000027 \ A023194.

a(n) ~ n. - Charles R Greathouse IV, May 27 2011

Example

a(1)=1 because A000203(1) is not prime. a(2)=3 because A000203(3)=3 is not prime. a(3)=5 because sigma(5)=6 is not prime.

Mathematica

Select[Range[1000], ! PrimeQ[DivisorSigma[1, #]]&] (* Vincenzo Librandi, Jan 18 2013 *)

Prog

(Magma) [n: n in [1..80]| not IsPrime(SumOfDivisors(n))]; // Vincenzo Librandi, Jan 18 2013
(PARI) isok(k) = !isprime(sigma(k)); \\ Michel Marcus, Mar 10 2022

Crossrefs

Complement of A023194.

Cf. A000203.

Sequence in context: A254130 A114978 A138891 * A181744 A343217 A160685

Adjacent sequences: A163159 A163160 A163161 * A163163 A163164 A163165

Keyword

nonn,easy

Author

Juri-Stepan Gerasimov, Jul 22 2009

Extensions

Edited by R. J. Mathar, Jul 25 2009

Status

approved