A308150 - OEIS

%I #15 May 15 2019 11:48:57

%S 4,8,18,20,21,27,32,35,36,39,50,55,57,63,65,77,85,98,100,104,111,115,

%T 125,128,129,155,161,171,175,185,187,189,196,201,203,205,209,221,235,

%U 237,242,245,265,275,279,291,299,305,309,319,323,324,325,327,335,338,341,365,371,377,381,385,391

%N Numbers k such that sigma(k) mod k is prime, where sigma = A000203.

%C Includes 1+A000668.

%H Robert Israel, <a href="/A308150/b308150.txt">Table of n, a(n) for n = 1..10000</a>

%e a(3) = 18 is in the sequence because sigma(18) = 39, 39 == 3 (mod 18), and 3 is prime.

%p select(n -> isprime(numtheory:-sigma(n) mod n), [$2..1000]);

%o (PARI) isok(n) = isprime(sigma(n) % n); \\ _Michel Marcus_, May 15 2019

%Y Cf. A000203, A000668, A054024.

%Y Includes A037020 and A075081.

%K nonn

%O 1,1

%A _J. M. Bergot_ and _Robert Israel_, May 14 2019