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 A199903 #8 Jun 02 2025 05:25:07 %S A199903 1,4,12,24,82,232,654,2044,6096,18564,57500,177032,550098,1715956, %T A199903 5359158,16799508,52760640,165976252,523094304,1650781728,5216112586, %U A199903 16501009804,52252555938,165618780196,525388548976,1667965025692 %N A199903 Number of -2..2 arrays x(0..n-1) of n elements with zero sum, and adjacent elements not equal modulo three (with -1 modulo 3 = 2). %C A199903 Column 2 of A199909 %H A199903 R. H. Hardin, <a href="/A199903/b199903.txt">Table of n, a(n) for n = 1..200</a> %e A199903 Some solutions for n=6 %e A199903 ..1....0....0....1....1....1....1....1...-1...-1...-1....0....0...-2....1...-1 %e A199903 ..2....1...-2....2....0....0....0...-1....0....0...-2...-1...-2....2....2....0 %e A199903 .-2....0....2....0....2...-1...-1...-2....1....1....0....0...-1....0...-2....1 %e A199903 ..0....1....1...-1...-2...-2...-2....2....0....2....1....2....0...-1...-1...-1 %e A199903 .-1....0...-1....0...-1....0....2...-2...-1...-2....0....0....2....0...-2....0 %e A199903 ..0...-2....0...-2....0....2....0....2....1....0....2...-1....1....1....2....1 %K A199903 nonn %O A199903 1,2 %A A199903 _R. H. Hardin_ Nov 11 2011