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 A267903 #6 Aug 12 2016 06:54:16 %S A267903 0,6,20,268,2324,28318,315196,3786836,45099306,547824556,6687936028, %T A267903 82350672500,1019550923830,12691867518330,158695413996448, %U A267903 1992293553570646,25099547132728742,317210539882144928 %N A267903 Number of 3Xn arrays containing n copies of 0..3-1 with every element equal to at least one horizontal or vertical neighbor and the top left element equal to 0. %C A267903 Row 3 of A267901. %H A267903 R. H. Hardin, <a href="/A267903/b267903.txt">Table of n, a(n) for n = 1..41</a> %e A267903 Some solutions for n=4 %e A267903 ..0..2..1..1....0..0..2..2....0..0..1..1....0..2..2..1....0..0..1..1 %e A267903 ..0..2..0..2....2..2..1..0....2..1..1..0....0..0..0..1....2..0..1..1 %e A267903 ..1..1..0..2....1..1..1..0....2..2..2..0....1..1..2..2....2..0..2..2 %Y A267903 Cf. A267901. %K A267903 nonn %O A267903 1,2 %A A267903 _R. H. Hardin_, Jan 22 2016