%I #18 Mar 10 2022 07:53:36
%S 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,
%T 31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,
%U 54,55,56,57,58,59,60,61,62,63,65,66,67,68,69,70,71,72,73,74,75,76,77
%N Numbers k such that sigma(k) is not prime.
%H Vincenzo Librandi, <a href="/A163162/b163162.txt">Table of n, a(n) for n = 1..1000</a>
%F A000027 \ A023194.
%F a(n) ~ n. - _Charles R Greathouse IV_, May 27 2011
%e 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.
%t Select[Range[1000], ! PrimeQ[DivisorSigma[1, #]]&] (* _Vincenzo Librandi_, Jan 18 2013 *)
%o (Magma) [n: n in [1..80]| not IsPrime(SumOfDivisors(n))]; // _Vincenzo Librandi_, Jan 18 2013
%o (PARI) isok(k) = !isprime(sigma(k)); \\ _Michel Marcus_, Mar 10 2022
%Y Complement of A023194.
%Y Cf. A000203.
%K nonn,easy
%O 1,2
%A _Juri-Stepan Gerasimov_, Jul 22 2009
%E Edited by _R. J. Mathar_, Jul 25 2009
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