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 A212931 #6 Jul 22 2025 22:59:20 %S A212931 1,15,568,39695,4431876,724082352,163050236504,48442719007503, %T A212931 18368207542264060,8659014569831436140,4968837728849467193040, %U A212931 3410836618457710864892912,2760233360074280192888992948 %N A212931 Number of 2Xn 0..n-1 arrays with no column j greater than column j-1 in all rows. %C A212931 Row 2 of A212930 %H A212931 R. H. Hardin, <a href="/A212931/b212931.txt">Table of n, a(n) for n = 1..17</a> %e A212931 Some solutions for n=3 %e A212931 ..0..1..2....0..0..1....2..1..0....0..0..0....2..1..2....0..2..1....0..2..0 %e A212931 ..1..0..0....0..1..1....2..1..2....1..2..0....0..0..0....0..0..2....1..0..1 %K A212931 nonn %O A212931 1,2 %A A212931 _R. H. Hardin_ May 31 2012