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 A091531 #3 Mar 30 2012 17:22:32
%S A091531 7,23,31,43,59,67,71,73,101,103,107,127,131,137,139,179,199,211,223,
%T A091531 227,239,269,281,283,307,311,331,347,359,367,379,383,431,439,463,467,
%U A091531 479,487,491,503,523,547,563,571,607,619,631,643,659,661,683,691,719,727
%N A091531 Primes p such that k = 2p is the smallest positive solution to the equation phi(p+k) = phi(p) + phi(k), where phi is Euler's totient function.
%C A091531 Note that for all primes p > 3, phi(3p) = phi(p) + phi(2p).
%t A091531 lst={}; Do[p=Prime[n]; k=1; While[EulerPhi[p+k]!=EulerPhi[p]+EulerPhi[k], k++ ]; If[k==2p, AppendTo[lst, p]], {n, 3, 200}]; lst
%Y A091531 Cf. A066426 (least k such that phi(n+k)=phi(n)+phi(k)).
%K A091531 nonn
%O A091531 1,1
%A A091531 _T. D. Noe_, Jan 19 2004