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 A199399 #7 Mar 30 2012 18:37:32
%S A199399 1,4,29,96,529,3620,13101,66048,325601,1653380,9189501,40187552,
%T A199399 216562929,1143607140,5098589837,26660767488,161932208321,
%U A199399 635945960964,4313170161629,20567937335008,74804061006033,551572049610276,2689136522898669,10088864973699328,67578042506892449
%N A199399 Binary XOR of 5^k as k varies from 0 to n.
%e A199399 a(2) = 1 XOR 5 = 4; a(3) = 1 XOR 5 XOR 25 = 29; a(4) = 1 XOR 5 XOR 25 XOR 125 = 96.
%o A199399 (PARI) {a(n)=if(n<0,0,bitxor(a(n-1),5^n))}
%Y A199399 Cf. A199397.
%K A199399 nonn
%O A199399 0,2
%A A199399 _Paul D. Hanna_, Nov 05 2011