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 A067683 #12 Apr 20 2025 02:29:50
%S A067683 1,8,9,10,12,15,20,28,30,32,35,36,42,45,50,54,58,60,70,80,92,93,95,
%T A067683 100,110,114,122,123,124,125,130,132,142,143,145,152,155,162,165,168,
%U A067683 169,171,172,174,175,176,178,180,183,185,186,195,198,200,204,209,212,216
%N A067683 Numbers k such that phi(k)*sigma(k) + 1 is prime.
%H A067683 Amiram Eldar, <a href="/A067683/b067683.txt">Table of n, a(n) for n = 1..10000</a>
%t A067683 Select[Range[216], PrimeQ[EulerPhi[#]*DivisorSigma[1, #] + 1] &] (* _Amiram Eldar_, Apr 20 2025 *)
%o A067683 (PARI) isok(k) = {my(f = factor(k)); isprime(eulerphi(k) * sigma(k) + 1);} \\ _Amiram Eldar_, Apr 20 2025
%Y A067683 Cf. A000010 (phi), A000203 (sigma), A062354.
%K A067683 nonn,easy
%O A067683 1,2
%A A067683 _Benoit Cloitre_, Feb 04 2002