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 A195638 #12 May 17 2020 01:40:51 %S A195638 1,1,3,19,391,25532,5539434,4116290300,10699755803977, %T A195638 98834805799836193,3285452504354615242708,397045217438108647483964418, %U A195638 175907525023476787251523714204914,287726899459800056272961335544498108972,1747931723877860528675045077016711589788124993 %N A195638 Number of lower triangles of an n X n 0..n array with all row sums equal to the length of the row and all column sums equal to the length of the column. %e A195638 Some solutions for n=4: %e A195638 ..1........1........1........1........1........1........1........1 %e A195638 ..2.0......0.2......2.0......0.2......2.0......1.1......1.1......2.0 %e A195638 ..1.1.1....1.1.1....0.2.1....2.1.0....1.2.0....0.2.1....2.1.0....1.0.2 %e A195638 ..0.2.1.1..2.0.1.1..1.1.1.1..1.0.2.1..0.1.2.1..2.0.1.1..0.1.2.1..0.3.0.1 %Y A195638 Diagonal of A195644. %K A195638 nonn %O A195638 1,3 %A A195638 _R. H. Hardin_, Sep 21 2011 %E A195638 a(12)-a(15) from _Andrew Howroyd_, May 16 2020