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 A218709 #4 Nov 05 2012 12:18:07
%S A218709 0,5,70,239,239,143044,1999509,6826318,6826318,822557039,52199939826,
%T A218709 603633907222,11356596271444,11356596271444,1828607235824962,
%U A218709 13920898306972194,13920898306972194,2675587335039691558,49226908181248336040,513050126578538629605
%N A218709 a(n) is smallest number such that a(n)^2 + 1 is divisible by 13^n.
%e A218709 a(4) = 239 because 239^2+1 = 2*13^4.
%t A218709 b=5;n13=13;jo=Join[{0,b},Table[n13=13*n13;b=PowerMod[b,13,n13];b=Min[b,n13-b],{99}]]
%Y A218709 Cf. A002522, A049532, A034939.
%K A218709 nonn
%O A218709 0,2
%A A218709 _Michel Lagneau_, Nov 04 2012