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 A222839 #12 Mar 31 2025 22:00:50 %S A222839 1,8,105,1585,23107,353631,5338646,81874696,1252153841,19270376854, %T A222839 296641979113,4580055753005,70780702170260,1095722113681548, %U A222839 16978656325074532,263390892198118920,4089414898741050209,63545865459514492673,988136631583140164460,15375690197520184535506 %N A222839 Number of n X 2 0..3 arrays with no more than n elements equal to at least one horizontal or vertical neighbor, with new values introduced in row major 0..3 order. %H A222839 R. H. Hardin, <a href="/A222839/b222839.txt">Table of n, a(n) for n = 1..210</a> %e A222839 Some solutions for n=3: %e A222839 ..0..1....0..1....0..1....0..1....0..1....0..1....0..1....0..1....0..1....0..0 %e A222839 ..2..3....1..2....1..0....2..3....2..3....2..3....2..0....0..0....2..2....1..2 %e A222839 ..1..3....2..3....0..1....0..3....1..0....3..1....0..1....2..1....2..3....2..0 %Y A222839 Column 2 of A222842. %K A222839 nonn %O A222839 1,2 %A A222839 _R. H. Hardin_, Mar 06 2013