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 A067720 #18 Nov 21 2020 14:01:32 %S A067720 1,2,4,6,8,10,16,36,40,66,126,130,150,156,180,210,240,250,256,270,280, %T A067720 306,396,400,420,430,466,490,556,570,576,646,690,700,750,760,826,906, %U A067720 910,936,946,966,1060,1096,1150,1276,1290,1306,1320,1366,1566,1570 %N A067720 Numbers k such that phi(k^2+1) = k*phi(k+1). %C A067720 a(n)+1 is prime except for a(5)=8. %C A067720 Superset of A070689. Is a(5)=8 the only additional value? - _Ralf Stephan_, Feb 11 2004 %H A067720 Amiram Eldar, <a href="/A067720/b067720.txt">Table of n, a(n) for n = 1..10000</a> %t A067720 Select[Range[2000], EulerPhi[#^2 + 1] == #*EulerPhi[# + 1] &] (* _Amiram Eldar_, Nov 21 2020 *) %o A067720 (PARI) isok(k) = eulerphi(k^2+1) == k*eulerphi(k+1); \\ _Michel Marcus_, Nov 21 2020 %Y A067720 Cf. A000010, A070689. %K A067720 nonn %O A067720 1,2 %A A067720 _Benoit Cloitre_, Feb 05 2002