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 A268988 #7 Apr 19 2020 23:45:24 %S A268988 2,13,174,6009,421450,66954420,22995344760,17373640598542, %T A268988 29082748697180602,107300241515211644436,883523091936988991189604, %U A268988 16140842052133986432421342048,659806253617230823177640504040304 %N A268988 Number of n X n binary arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two not more than once. %C A268988 Diagonal of A268995. %H A268988 R. H. Hardin, <a href="/A268988/b268988.txt">Table of n, a(n) for n = 1..26</a> %e A268988 Some solutions for n=4 %e A268988 ..0..1..0..1. .0..0..1..0. .1..1..0..1. .0..0..0..1. .0..0..0..0 %e A268988 ..0..1..0..0. .0..0..0..0. .0..1..0..0. .0..1..0..0. .0..0..1..0 %e A268988 ..0..0..1..1. .1..0..0..0. .0..0..1..0. .0..0..0..1. .1..0..1..0 %e A268988 ..0..0..0..0. .0..0..1..1. .0..0..0..1. .0..1..0..0. .1..0..0..0 %Y A268988 Cf. A268995. %K A268988 nonn %O A268988 1,1 %A A268988 _R. H. Hardin_, Feb 17 2016