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 A268880 #7 Apr 19 2020 23:44:26 %S A268880 0,5,84,4133,317966,55491832,19876401224,15564645769042, %T A268880 26617222097087230,99905925686618815012,832547140418699137884096, %U A268880 15355865014207234684746438192,632296902429868672596327438625296 %N A268880 Number of n X n binary arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two exactly once. %C A268880 Diagonal of A268886. %H A268880 R. H. Hardin, <a href="/A268880/b268880.txt">Table of n, a(n) for n = 1..26</a> %e A268880 Some solutions for n=4 %e A268880 ..1..1..0..0. .0..1..0..1. .0..0..0..1. .0..0..1..0. .0..0..0..0 %e A268880 ..0..0..0..0. .0..1..0..0. .0..0..1..0. .1..0..1..0. .0..1..0..1 %e A268880 ..0..0..0..0. .1..0..0..1. .1..0..1..0. .0..0..0..0. .0..1..0..0 %e A268880 ..0..0..0..1. .0..0..0..1. .1..0..0..1. .1..0..1..1. .1..0..1..0 %Y A268880 Cf. A268886. %K A268880 nonn %O A268880 1,2 %A A268880 _R. H. Hardin_, Feb 15 2016