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 A199240 #7 Jul 22 2025 13:15:13 %S A199240 1,2,2,12,36,39,288,836,888,7380,22149,23706,201515,611372,654468, %T A199240 5704006,17499974,18760333,166063522,513160156,550594928,4932198432, %U A199240 15327377362,16456851660,148765279666,464332349724,498808175988 %N A199240 Number of nX2 0..2 arrays with values 0..2 introduced in row major order, the number of instances of each value within one of each other, and every element equal to zero or two horizontal or vertical neighbors. %C A199240 Column 2 of A199246 %H A199240 R. H. Hardin, <a href="/A199240/b199240.txt">Table of n, a(n) for n = 1..154</a> %e A199240 Some solutions for n=4 %e A199240 ..0..1....0..1....0..1....0..1....0..1....0..1....0..1....0..1....0..1....0..1 %e A199240 ..1..2....1..2....1..2....2..0....2..0....2..0....2..0....1..0....2..0....1..0 %e A199240 ..2..0....2..1....0..1....1..2....0..2....1..2....0..1....0..2....1..2....2..1 %e A199240 ..1..2....0..2....2..0....2..1....2..1....0..1....1..2....2..1....2..0....0..2 %K A199240 nonn %O A199240 1,2 %A A199240 _R. H. Hardin_ Nov 04 2011