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 A050530 #20 Sep 08 2022 08:44:58 %S A050530 4,9,15,25,33,35,49,51,65,77,87,91,95,119,121,123,143,161,169,177,185, %T A050530 209,213,215,217,221,247,255,259,287,289,303,321,329,335,341,361,371, %U A050530 377,395,403,407,411,427,435,437,447,455,469,473,485,511,515,527,529 %N A050530 Numbers k such that k - phi(k) is prime. %C A050530 If k = p^2 is the square of a prime, then p^2 - phi(p^2) = p, so this sequence is infinite and generates all primes. %C A050530 No prime p is a term of this sequence because A051953(p)=1. Other cases exist; e.g., k - phi(k) = 23 if k = 95, 119, 143, 529. %H A050530 Amiram Eldar, <a href="/A050530/b050530.txt">Table of n, a(n) for n = 1..10000</a> %F A050530 Numbers k such that A051953(k) is prime. %t A050530 Select[Range[600],PrimeQ[#-EulerPhi[#]]&] (* _Harvey P. Dale_, Jun 23 2013 *) %o A050530 (Magma) [n: n in [1..600] | IsPrime(n-EulerPhi(n))]; // _Vincenzo Librandi_, Dec 18 2015 %Y A050530 Cf. A051953, A005278. %K A050530 nonn %O A050530 1,1 %A A050530 _Labos Elemer_, Dec 29 1999