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A163162
Numbers k such that sigma(k) is not prime.
2
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
FORMULA
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
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
Edited by R. J. Mathar, Jul 25 2009
STATUS
approved