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 A211909 #6 Jun 02 2025 07:51:33 %S A211909 0,0,42,11296,3997836,350026718,12075054829,228741733963 %N A211909 Number of lower triangular n X n arrays colored with integers 0 upwards introduced in row major order, with no element equal to any horizontal or vertical neighbor, and containing the value n(n+1)/2-6. %C A211909 Column 5 of A211910 %e A211909 Some solutions for n=4 %e A211909 ..0........0........0........0........0........0........0........0 %e A211909 ..1.2......1.2......1.2......1.2......1.2......1.0......1.2......1.2 %e A211909 ..2.3.2....0.3.0....0.3.4....3.4.3....2.1.3....2.3.2....2.3.2....3.0.4 %e A211909 ..4.1.4.0..4.5.2.6..3.0.2.0..5.0.5.3..4.0.4.3..4.2.0.1..3.4.3.1..4.5.2.0 %K A211909 nonn %O A211909 1,3 %A A211909 _R. H. Hardin_ Apr 25 2012