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 A251142 #10 Aug 11 2015 09:18:11 %S A251142 13,49,143,401,1061,2763,7101,18193,46575,119441,307045,791499,2045725 %N A251142 Number of (n+1) X (n+1) 0..2 arrays with every 2 X 2 subblock summing to 4 and no 2 X 2 subblock having exactly two nonzero entries. %C A251142 Diagonal of A251149. %e A251142 Some solutions for n=4: %e A251142 ..1..0..1..1..1....1..1..1..1..1....0..1..0..2..1....1..1..0..1..1 %e A251142 ..2..1..2..0..2....1..1..1..1..1....1..2..1..1..0....1..1..2..1..1 %e A251142 ..1..0..1..1..1....0..2..0..2..0....0..1..0..2..1....2..0..1..0..2 %e A251142 ..1..2..1..1..1....1..1..1..1..1....1..2..1..1..0....1..1..2..1..1 %e A251142 ..0..1..0..2..0....1..1..1..1..1....1..0..1..1..2....1..1..0..1..1 %Y A251142 Cf. A251149. %K A251142 nonn %O A251142 1,1 %A A251142 _R. H. Hardin_, Nov 30 2014