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 A195516 #7 Jul 22 2025 12:44:24 %S A195516 1,1,5,65,3753,860017,839301197,3535646416019,65638445025610187, %T A195516 5443595184898837903553 %N A195516 Number of lower triangles of an n X n -2..2 array with all row and column sums zero. %C A195516 Column 2 of A195522 %e A195516 Some solutions for n=4 %e A195516 ..0........0........0........0........0........0........0........0 %e A195516 .-1.1.....-2.2.....-2.2......1-1......1-1......0.0......2-2......1-1 %e A195516 ..0-2.2....1.0-1....2.0-2...-1-1.2....1-1.0...-1.2-1....0.2-2....0.0.0 %e A195516 ..1.1-2.0..1-2.1.0..0-2.2.0..0.2-2.0.-2.2.0.0..1-2.1.0.-2.0.2.0.-1.1.0.0 %K A195516 nonn %O A195516 1,3 %A A195516 _R. H. Hardin_ Sep 20 2011