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 A117284 #13 Mar 23 2021 05:20:07 %S A117284 1,2,3,5,7,9,10,11,13,17,19,23,29,31,37,39,41,43,45,47,52,53,54,55,57, %T A117284 59,61,67,68,71,73,79,83,85,89,90,97,101,103,107,109,113,114,117,127, %U A117284 129,131,137,139,149,151,157,159,163,167,168,173,175,179,181,191,193 %N A117284 Numbers k for which the cototient k-phi(k) is a triangular number. %H A117284 Amiram Eldar, <a href="/A117284/b117284.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1001 from Harvey P. Dale) %e A117284 39 is in the sequence because 39-phi(39) = 15, which is a triangular number. %t A117284 Select[Range[200],OddQ[Sqrt[8(#-EulerPhi[#])+1]]&] (* _Harvey P. Dale_, Jun 29 2015 *) %o A117284 (PARI) isok(n) = ispolygonal(n - eulerphi(n), 3); \\ _Michel Marcus_, Feb 26 2014 %Y A117284 Cf. A000010, A000217, A051953. %Y A117284 Cf. A117283, A117285, A117286, A117287, A117288, A117289. %K A117284 nonn %O A117284 1,2 %A A117284 Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006 %E A117284 Offset corrected by _Amiram Eldar_, Mar 23 2021