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 A117285 #13 Mar 23 2021 05:19:26 %S A117285 1,2,3,5,7,11,13,17,18,19,20,22,23,25,29,30,31,37,41,43,47,53,59,61, %T A117285 67,71,73,75,79,83,89,97,101,102,103,107,109,110,113,127,131,132,137, %U A117285 139,140,149,151,155,157,163,167,173,179,181,191,193,197,199,203,211,223 %N A117285 Numbers k for which the cototient k-phi(k) is a pentagonal number. %H A117285 Amiram Eldar, <a href="/A117285/b117285.txt">Table of n, a(n) for n = 1..10000</a> %e A117285 30 is in the sequence because 30-phi(30) = 22, which is a pentagonal number. %t A117285 pentQ[n_] := n == 0 || IntegerQ[(Sqrt[24*n + 1] + 1)/6]; Select[Range[250], pentQ[# - EulerPhi[#]] &] (* _Amiram Eldar_, Mar 23 2021 *) %o A117285 (PARI) isok(n) = ispolygonal(n - eulerphi(n), 5); \\ _Michel Marcus_, Feb 26 2014 %Y A117285 Cf. A000010, A000326, A051953. %Y A117285 Cf. A117283, A117284, A117286, A117287, A117288, A117289. %K A117285 nonn %O A117285 1,2 %A A117285 Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006 %E A117285 Offset corrected by _Amiram Eldar_, Mar 23 2021