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 A208550 #9 May 07 2021 13:33:55 %S A208550 2,16,90,1089,10080,150544,1913940,33930625,544003170,11177987076, %T A208550 215177010048,5033386477441,112708053008152,2958949185458176, %U A208550 75387859827570360,2195709810260687361,62623865365172370000 %N A208550 Number of n X n 0..1 arrays avoiding 0 0 0 and 0 0 1 horizontally and 0 0 1 and 0 1 1 vertically. %C A208550 Diagonal of A208555. %H A208550 R. H. Hardin, <a href="/A208550/b208550.txt">Table of n, a(n) for n = 1..38</a> %e A208550 Some solutions for n=4 %e A208550 ..1..0..1..0....0..1..1..0....0..1..1..1....0..1..0..0....1..0..1..1 %e A208550 ..0..1..1..1....0..1..1..0....0..1..0..1....1..0..1..1....0..1..1..0 %e A208550 ..1..0..1..0....0..1..0..0....0..1..1..0....0..1..0..0....1..0..1..1 %e A208550 ..0..1..0..1....0..1..1..0....0..1..0..0....1..0..1..1....0..1..1..0 %Y A208550 Cf. A208555. %K A208550 nonn %O A208550 1,1 %A A208550 _R. H. Hardin_ Feb 28 2012