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 A209219 #9 Jul 22 2025 21:40:49 %S A209219 2,16,60,196,900,5476,38232,287296,3132576,37625956,515992736, %T A209219 9773299600,197179936872,5340055452736,166631085360000, %U A209219 6057724646680996,305548384194976300,15689243192533764624,1137621414073872082528 %N A209219 Number of n X n 0..1 arrays avoiding 0 0 0 and 0 1 0 horizontally and 0 1 0 and 1 1 1 vertically. %C A209219 Diagonal of A209224 %H A209219 R. H. Hardin, <a href="/A209219/b209219.txt">Table of n, a(n) for n = 1..21</a> %e A209219 Some solutions for n=4 %e A209219 ..0..0..1..1....0..1..1..0....1..1..0..1....1..0..0..1....1..1..0..0 %e A209219 ..1..1..0..0....0..1..1..0....0..1..1..0....0..1..1..0....0..1..1..1 %e A209219 ..1..1..1..0....1..0..0..1....1..0..1..1....1..1..1..1....1..0..1..1 %e A209219 ..0..0..1..1....1..1..0..1....1..1..0..1....1..0..0..1....1..1..0..0 %K A209219 nonn %O A209219 1,1 %A A209219 _R. H. Hardin_ Mar 06 2012