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 A199242 #7 Jul 22 2025 13:15:26 %S A199242 3,12,25,440,2907,7100,134821,934004,2277108,46296108,330908530, %T A199242 820702285,17258404593,125752465794,315014793177,6765057712760, %U A199242 49918706666467,125910537922412,2743415189299833,20425391744058298 %N A199242 Number of nX4 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 A199242 Column 4 of A199246 %H A199242 R. H. Hardin, <a href="/A199242/b199242.txt">Table of n, a(n) for n = 1..30</a> %e A199242 Some solutions for n=5 %e A199242 ..0..1..2..1....0..1..0..2....0..1..0..1....0..0..1..0....0..1..0..1 %e A199242 ..2..0..1..2....2..2..1..0....1..2..1..2....0..0..2..1....1..0..1..2 %e A199242 ..0..2..0..1....2..2..0..1....2..1..2..0....1..2..1..0....0..2..0..1 %e A199242 ..2..0..1..0....0..1..1..2....0..2..0..2....2..1..2..1....2..0..2..0 %e A199242 ..1..2..0..1....2..1..1..0....1..0..2..0....1..2..0..2....1..2..1..2 %K A199242 nonn %O A199242 1,1 %A A199242 _R. H. Hardin_ Nov 04 2011