This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A308150 #15 May 15 2019 11:48:57 %S A308150 4,8,18,20,21,27,32,35,36,39,50,55,57,63,65,77,85,98,100,104,111,115, %T A308150 125,128,129,155,161,171,175,185,187,189,196,201,203,205,209,221,235, %U A308150 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 A308150 Numbers k such that sigma(k) mod k is prime, where sigma = A000203. %C A308150 Includes 1+A000668. %H A308150 Robert Israel, <a href="/A308150/b308150.txt">Table of n, a(n) for n = 1..10000</a> %e A308150 a(3) = 18 is in the sequence because sigma(18) = 39, 39 == 3 (mod 18), and 3 is prime. %p A308150 select(n -> isprime(numtheory:-sigma(n) mod n), [$2..1000]); %o A308150 (PARI) isok(n) = isprime(sigma(n) % n); \\ _Michel Marcus_, May 15 2019 %Y A308150 Cf. A000203, A000668, A054024. %Y A308150 Includes A037020 and A075081. %K A308150 nonn %O A308150 1,1 %A A308150 _J. M. Bergot_ and _Robert Israel_, May 14 2019