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 A199241 #7 Jul 22 2025 13:15:20 %S A199241 1,2,6,25,110,472,2080,9279,41704,187500,853012,3919309,18163022, %T A199241 84589504,395762792,1858908001,8762833946,41439737234,196527899276, %U A199241 934460643457,4453641597888,21271612632536,101795771318046 %N A199241 Number of nX3 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 A199241 Column 3 of A199246 %H A199241 R. H. Hardin, <a href="/A199241/b199241.txt">Table of n, a(n) for n = 1..64</a> %e A199241 Some solutions for n=4 %e A199241 ..0..1..2....0..1..2....0..1..2....0..1..2....0..1..0....0..1..2....0..1..0 %e A199241 ..1..2..0....2..0..1....1..2..0....2..0..1....1..0..1....1..2..0....2..0..1 %e A199241 ..2..0..1....1..2..0....0..1..2....0..1..2....2..2..0....2..0..1....1..2..0 %e A199241 ..0..1..2....2..0..1....2..0..1....2..0..1....2..2..1....1..2..0....2..1..2 %K A199241 nonn %O A199241 1,2 %A A199241 _R. H. Hardin_ Nov 04 2011