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 A209512 #9 Oct 06 2015 21:46:00 %S A209512 57,4995,2071311,4099186545,38883961392081,1772313933013227273, %T A209512 388756746462464273412765,410796134340987241848792211659, %U A209512 2092643007412378548345728130194640495 %N A209512 Number of (n+1) X (n+1) 0..2 arrays with the number of clockwise edge increases in every 2 X 2 subblock equal to the number of counterclockwise edge increases. %C A209512 Diagonal of A209517. %H A209512 R. H. Hardin, <a href="/A209512/b209512.txt">Table of n, a(n) for n = 1..10</a> %e A209512 Some solutions for n=4: %e A209512 ..1..2..0..0..1....2..1..0..2..2....1..0..0..0..0....0..1..2..2..0 %e A209512 ..0..1..2..2..0....0..2..1..0..0....1..0..1..0..0....0..0..1..1..2 %e A209512 ..0..1..1..1..2....1..0..2..1..0....2..1..0..2..2....2..0..1..0..1 %e A209512 ..1..1..2..1..1....2..1..0..2..1....2..1..1..0..2....0..2..0..1..2 %e A209512 ..1..2..2..2..1....2..1..1..0..2....2..1..1..0..2....0..0..2..0..1 %K A209512 nonn %O A209512 1,1 %A A209512 _R. H. Hardin_, Mar 09 2012