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 A056638 #6 Mar 30 2012 16:48:51 %S A056638 2,122,3400,64210,959742,12294090,141329502,1500596864,14998956452, %T A056638 143004562082,1312929589140,11688960621918,101451353794222, %U A056638 861927965928610,7191518458613614,59078731187602024 %N A056638 Number of polygons of length 2n with two (self-avoiding polygon) holes on square lattice (not allowing rotations). %H A056638 I. Jensen, <a href="/A056638/b056638.txt">Table of n, a(n) for n = 12..43</a> (from link below) %H A056638 I. Jensen, <a href="http://www.ms.unimelb.edu.au/~iwan/polygons/series/square.perim.2hole.ser">More terms</a> %Y A056638 Cf. A002931. %K A056638 nonn %O A056638 12,1 %A A056638 _N. J. A. Sloane_, Aug 30 2000