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 A039689 #17 Jun 28 2021 04:10:09 %S A039689 15,16,20,24,25,30,33,35,39,44,45,50,51,52,56,64,65,66,68,69,70,72,78, %T A039689 80,81,84,85,87,90,92,96,102,104,105,112,116,120,121,123,128,129,130, %U A039689 136,138,140,141,143,144,147,155,156,159,160,161,162,164,165,168,170 %N A039689 Numbers k such that phi(k) + 1 is not a prime. %H A039689 Antti Karttunen, <a href="/A039689/b039689.txt">Table of n, a(n) for n = 1..39340</a> %e A039689 phi(20)+1 = 8+1 = 9 is not prime. %t A039689 Select[Range[200],!PrimeQ[EulerPhi[#]+1]&] (* _Harvey P. Dale_, Aug 31 2018 *) %o A039689 (PARI) isok(k) = !isprime(eulerphi(k)+1); \\ _Michel Marcus_, Jun 28 2021 %Y A039689 Cf. A000010, A007614, A039649, A039698 (complement). %Y A039689 Positions of zeros in A296079. %Y A039689 Cf. also A263029. %K A039689 nonn,easy %O A039689 1,1 %A A039689 _Olivier Gérard_ %E A039689 Name edited by _Antti Karttunen_, Dec 05 2017