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 A195639 #9 May 17 2020 01:41:17 %S A195639 1,1,3,19,379,22506,4063437,2303397986,4165684145897, %T A195639 24351094470461293,465231630106636264261,29310412705455253400706632, %U A195639 6135104108066245513806738233622,4293669524764808720927311131475805624,10102086274414407316013385512463577654333437 %N A195639 Number of lower triangles of an n X n 0..3 array with all row sums equal to the length of the row and all column sums equal to the length of the column. %e A195639 Some solutions for n=4: %e A195639 ..1........1........1........1........1........1........1........1 %e A195639 ..0.2......0.2......1.1......2.0......2.0......0.2......0.2......1.1 %e A195639 ..0.1.2....3.0.0....2.0.1....1.2.0....1.0.2....2.1.0....3.0.0....2.0.1 %e A195639 ..3.0.0.1..0.1.2.1..0.2.1.1..0.1.2.1..0.3.0.1..1.0.2.1..0.1.2.1..0.2.1.1 %Y A195639 Column 3 of A195644. %K A195639 nonn %O A195639 1,3 %A A195639 _R. H. Hardin_, Sep 21 2011 %E A195639 a(12)-a(15) from _Andrew Howroyd_, May 16 2020