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 A094919 #9 Feb 18 2023 22:48:51
%S A094919 0,1,0,1,1,3,1,5,0,5,1,9,1,5,6,5,1,9,1,5,0,5,1,21,16,5,0,1,1,15,1,21,
%T A094919 21,5,26,9,1,5,21,5,1,21,1,41,36,5,1,21,22,25,21,33,1,27,11,5,21,5,1,
%U A094919 45,1,5,0,21,16,45,1,17,21,15,1,45,1,5,66,9,5,39,1,5,0,5,1,21,1,5,21,21,1,45
%N A094919 a(n) = (4^n-1)/3 mod n.
%H A094919 Antti Karttunen, <a href="/A094919/b094919.txt">Table of n, a(n) for n = 1..20000</a>
%t A094919 Table[Mod[(4^n-1)/3,n],{n,100}] (* _Harvey P. Dale_, Apr 11 2013 *)
%o A094919 (PARI) A094919(n) = ((((4^n)-1)/3)%n); \\ _Antti Karttunen_, Feb 18 2023
%Y A094919 Cf. also A094922.
%K A094919 nonn
%O A094919 1,6
%A A094919 _N. J. A. Sloane_, Jun 18 2004