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 A138654 #5 Aug 26 2019 13:21:37
%S A138654 0,3,1,23,97,423,2961,4343,63169,245575,408305,1809559,84001,50919655,
%T A138654 155110993,548906039,2768600449,15085235847,36877174193,258140219351,
%U A138654 707469907681,2753266098215,10476769665297,20560829523831,73557062489153,233424460713415
%N A138654 7^n mod 4^n.
%t A138654 Table[PowerMod[7,n,4^n],{n,0,30}] (* _Harvey P. Dale_, Aug 26 2019 *)
%K A138654 nonn
%O A138654 0,2
%A A138654 _N. J. A. Sloane_, May 20 2008