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 A213956 #6 Jun 02 2025 08:04:08 %S A213956 3,42,3186,1299735,2860343784,34107691901433,2211749000480369037, %T A213956 782349734245964422491828 %N A213956 Number of n X n 0..2 matrices with row and column i=1..n having sum <= i*2. %C A213956 Column 2 of A213962 %e A213956 Some solutions for n=4 %e A213956 ..0..2..0..0....2..0..0..0....2..0..0..0....2..0..0..0....2..0..0..0 %e A213956 ..0..1..2..0....0..1..0..0....0..1..1..0....0..2..2..0....0..1..0..1 %e A213956 ..1..0..2..0....0..1..1..0....0..1..0..2....0..0..1..1....0..2..1..0 %e A213956 ..0..1..1..2....0..1..2..2....0..2..0..1....0..0..2..0....0..1..0..0 %K A213956 nonn %O A213956 1,1 %A A213956 _R. H. Hardin_ Jun 28 2012