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 A117287 #13 Mar 23 2021 05:19:25
%S A117287 1,2,3,5,7,11,13,15,17,19,23,29,31,34,37,41,43,47,49,53,59,61,67,71,
%T A117287 73,79,83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,159,
%U A117287 163,167,173,175,179,181,189,191,193,196,197,199,208,211,223,227,229,233
%N A117287 Numbers k for which the cototient k-phi(k) is a heptagonal number.
%H A117287 Amiram Eldar, <a href="/A117287/b117287.txt">Table of n, a(n) for n = 1..10000</a>
%e A117287 49 is in the sequence because 49-phi(49) = 7, which is a heptagonal number.
%t A117287 hnQ[n_]:=Module[{c=n-EulerPhi[n]},IntegerQ[(3+Sqrt[9+40c])/10]]; Join[{1},Select[Range[0,250],hnQ]] (* _Harvey P. Dale_, Mar 26 2011 *)
%o A117287 (PARI) isok(n) = ispolygonal(n - eulerphi(n), 7); \\ _Michel Marcus_, Feb 26 2014
%Y A117287 Cf. A000010, A000566, A051953.
%Y A117287 Cf. A117283, A117284, A117285, A117286, A117288, A117289.
%K A117287 nonn
%O A117287 1,2
%A A117287 Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
%E A117287 Offset corrected by _Amiram Eldar_, Mar 23 2021