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 A203536 #11 Jun 02 2025 07:36:01 %S A203536 0,4,4,16,64,196,676,2304,7744,26244,88804,300304,1016064,3437316, %T A203536 11628100,39337984,133079296,450203524,1523028676,5152368400, %U A203536 17430336576,58966408900,199481929956,674842534144,2282975946304,7723252297476 %N A203536 Number of nX2 0..2 arrays with every element neighboring horizontally or vertically both a 0 and a 1. %C A203536 Column 2 of A203542 %C A203536 (A135491(n-2))^2 for n>=3 %H A203536 R. H. Hardin, <a href="/A203536/b203536.txt">Table of n, a(n) for n = 1..210</a> %F A203536 Empirical: a(n) = 2*a(n-1) +3*a(n-2) +6*a(n-3) -a(n-4) -a(n-6) %e A203536 Some solutions for n=5 %e A203536 ..0..1....1..1....0..0....1..0....0..0....0..1....0..0....0..1....1..0....1..1 %e A203536 ..0..1....0..0....1..1....1..0....1..1....0..1....1..1....0..1....1..0....0..0 %e A203536 ..2..1....1..2....1..2....0..2....2..0....1..2....2..2....0..1....0..0....2..1 %e A203536 ..0..0....1..0....0..0....0..1....1..0....1..0....0..0....0..0....0..1....0..1 %e A203536 ..1..1....1..0....1..1....0..1....1..0....1..0....1..1....1..1....0..1....0..1 %K A203536 nonn %O A203536 1,2 %A A203536 _R. H. Hardin_ Jan 02 2012