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 A057919 #14 Jul 13 2019 05:32:36 %S A057919 1,2,3,4,6,12,15,16,18,36,72,90,96,104,108,154,162,164,192,194,255, %T A057919 256,286,364,432,486,495,576,584,702,768,792,804,924,975,1066,1152, %U A057919 1260,1296,1458,2146,2204,2592,2625,2834,2916,3255,3382,3456,3705,3888 %N A057919 Numbers k such that phi(k) divides phi(k+1), where phi(k) is the Euler totient function A000010. %C A057919 The intersection of this sequence and A057920 is A001274. - _Michel Marcus_, Sep 14 2015 %H A057919 Amiram Eldar, <a href="/A057919/b057919.txt">Table of n, a(n) for n = 1..6255</a> (terms below 10^10) %e A057919 6 is included because phi(6) = 2 divides phi(7) = 6. %t A057919 Select[Range[4000], Divisible[EulerPhi[# + 1], EulerPhi[#]] &] (* _Amiram Eldar_, Jul 13 2019 *) %o A057919 (PARI) lista(nn) = for (n=1, nn, if (eulerphi(n+1) % eulerphi(n) == 0, print1(n, ", "))); \\ _Michel Marcus_, Sep 14 2015 %Y A057919 Cf. A001274, A057920. %K A057919 nonn %O A057919 1,2 %A A057919 _Leroy Quet_, Nov 11 2000 %E A057919 Offset set to 1 by _Michel Marcus_, Sep 14 2015