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 A268734 #7 Apr 19 2020 23:41:45 %S A268734 0,4,80,2876,200204,29134076,9039552112,6084192150856, %T A268734 8965720251221240,29122359898196696856,209490003763160695865576, %U A268734 3348908247850171650090422060,119286836904111393911302107179212 %N A268734 Number of n X n binary arrays with some 1 horizontally or vertically adjacent to some other 1 exactly once. %C A268734 Diagonal of A268740. %H A268734 R. H. Hardin, <a href="/A268734/b268734.txt">Table of n, a(n) for n = 1..26</a> %e A268734 Some solutions for n=4 %e A268734 ..0..0..0..0. .1..0..1..0. .0..1..0..1. .0..0..1..0. .0..0..1..0 %e A268734 ..0..0..0..1. .1..0..0..1. .0..0..1..0. .1..0..1..0. .0..0..0..0 %e A268734 ..1..0..0..0. .0..1..0..0. .0..0..1..0. .0..0..0..0. .0..0..1..0 %e A268734 ..1..0..0..0. .0..0..0..0. .0..1..0..1. .0..0..0..0. .1..1..0..0 %Y A268734 Cf. A268740. %K A268734 nonn %O A268734 1,2 %A A268734 _R. H. Hardin_, Feb 12 2016