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 A195515 #11 May 17 2020 01:42:06 %S A195515 1,1,3,15,199,6247,505623,105997283,58923059879,87882127554983, %T A195515 355824020816195749,3945677945542866439849, %U A195515 120747614641791880577007593,10263027079252827219477164325361,2436143827877272419846561427146618539,1622665432590314350761059585434700996826279 %N A195515 Number of lower triangles of an n X n -1..1 array with all row and column sums zero. %e A195515 Some solutions for 4 X 4: %e A195515 ..0........0........0........0........0........0........0........0 %e A195515 ..1-1.....-1.1......1-1......0.0.....-1.1......0.0.....-1.1......1-1 %e A195515 ..0.0.0....1-1.0....0.0.0....0.0.0....0.0.0....1.0-1....0.0.0...-1.1.0 %e A195515 .-1.1.0.0..0.0.0.0.-1.1.0.0..0.0.0.0..1-1.0.0.-1.0.1.0..1-1.0.0..0.0.0.0 %Y A195515 Column 1 of A195522 and column 2 of A195644. %K A195515 nonn %O A195515 1,3 %A A195515 _R. H. Hardin_, Sep 20 2011 %E A195515 a(13)-a(16) from _Andrew Howroyd_, May 16 2020