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 A269506 #6 Aug 12 2016 06:54:17 %S A269506 2,6,14,34,92,258,722,2022,5706,16232,46442,133418,384550,1111614, %T A269506 3221582,9357450,27233148,79394646,231822854,677835636,1984435484, %U A269506 5816254262,17064740572,50114985298,147303657830,433319078074 %N A269506 Number of 2Xn arrays containing n copies of 0..2-1 avoiding the pattern down-up in every row and column. %C A269506 Row 2 of A269505. %H A269506 R. H. Hardin, <a href="/A269506/b269506.txt">Table of n, a(n) for n = 1..210</a> %e A269506 Some solutions for n=4 %e A269506 ..1..0..0..1. .0..0..1..1. .0..1..1..0. .0..0..1..1. .0..0..1..0 %e A269506 ..0..1..1..0. .1..1..0..0. .1..1..0..0. .1..0..0..1. .0..1..1..1 %Y A269506 Cf. A269505. %K A269506 nonn %O A269506 1,1 %A A269506 _R. H. Hardin_, Feb 28 2016