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 A180643 #14 Mar 11 2020 09:57:17 %S A180643 1,2,3,4,5,6,7,8,9,10,11,12,14,17,18,22,25,32,33,34,35,37,39,40,41,44, %T A180643 45,48,50,52,55,56,57,60,63,66,67,70,72,74,75,76,78,81,82,84,87,88,90, %U A180643 100,108,110,114,116,121,126,127,129,131,132,134,143,147,150,151,155 %N A180643 Numbers n such that 1+phi(n)^2 is prime. Phi is the Euler totient function. %H A180643 Vincenzo Librandi, <a href="/A180643/b180643.txt">Table of n, a(n) for n = 1..2800</a> %e A180643 a(20)=34 since 1+phi(34)^2 = 1+16^2 = 257 is prime. %p A180643 select(t -> isprime(numtheory:-phi(t)^2+1), [$1..1000]); # _Robert Israel_, Mar 11 2020 %t A180643 Select[Range[200],PrimeQ[1+EulerPhi[#]^2]&] (* _Harvey P. Dale_, Aug 13 2014 *) %o A180643 (PARI) isok(n) = isprime(1 + eulerphi(n)^2) \\ _Michel Marcus_, Jul 18 2013 %Y A180643 Cf. A000010, A127435 (primes in this sequence). %K A180643 nonn %O A180643 1,2 %A A180643 _Carmine Suriano_, Sep 14 2010