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 A208137 #9 Jul 22 2025 21:11:16 %S A208137 2,16,108,900,6300,50176,352800,2722500,19111950,144288144,1010017008, %T A208137 7505103424,52382558592,384598425600,2677165603200,19473033306276, %U A208137 135229397960250,976281270250000,6765629202832500,48545357372612100 %N A208137 Number of n X n 0..1 arrays avoiding 0 0 1 and 0 1 1 horizontally and 0 0 1 and 1 0 1 vertically. %C A208137 Diagonal of A208142 %H A208137 R. H. Hardin, <a href="/A208137/b208137.txt">Table of n, a(n) for n = 1..70</a> %e A208137 Some solutions for n=4 %e A208137 ..1..1..1..1....0..0..0..0....1..0..0..0....0..0..0..0....1..1..0..1 %e A208137 ..1..1..1..1....1..0..1..0....0..1..0..0....0..1..0..0....1..1..0..1 %e A208137 ..1..1..1..1....0..0..0..0....0..1..0..0....0..0..0..0....0..1..0..0 %e A208137 ..1..1..1..1....0..0..0..0....0..0..0..0....0..0..0..0....0..0..0..0 %K A208137 nonn %O A208137 1,1 %A A208137 _R. H. Hardin_ Feb 23 2012