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 A117286 #15 Mar 23 2021 05:19:18
%S A117286 1,2,3,5,7,10,11,13,17,19,23,29,31,37,39,41,43,47,52,53,55,59,61,67,
%T A117286 71,73,79,83,89,90,97,101,103,107,109,113,117,127,129,131,137,139,149,
%U A117286 151,157,163,167,168,173,179,181,191,193,197,199,200,205,211,223,227,229
%N A117286 Numbers k for which the cototient k-phi(k) is a hexagonal number.
%C A117286 55 is in the sequence because 55-phi(55) = 15, which is a hexagonal number.
%H A117286 Amiram Eldar, <a href="/A117286/b117286.txt">Table of n, a(n) for n = 1..10000</a>
%t A117286 cthQ[n_]:=Module[{c=n-EulerPhi[n]},IntegerQ[(Sqrt[1+8c]+1)/4]]; Join[{1}, Select[Range[250],cthQ]] (* _Harvey P. Dale_, Jan 16 2012 *)
%o A117286 (PARI) isok(n) = ispolygonal(n - eulerphi(n), 6); \\ _Michel Marcus_, Feb 26 2014
%Y A117286 Cf. A000010, A000384, A051953.
%Y A117286 Cf. A117283, A117284, A117285, A117287, A117288, A117289.
%K A117286 nonn
%O A117286 1,2
%A A117286 Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
%E A117286 Corrected by _T. D. Noe_, Oct 25 2006