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 A199894 #7 Jul 22 2025 13:25:58 %S A199894 1,9,37,209,1043,5725,30199,164993,890299,4877477,26632013,146499083, %T A199894 805700567,4449304857,24590147861,136249639279,755738351775, %U A199894 4199211521573,23357174342185,130088542750275,725223072283995 %N A199894 Number of -4..4 arrays x(0..n-1) of n elements with zero sum, and adjacent elements not both strictly positive and not both strictly negative. %C A199894 Column 4 of A199898 %H A199894 R. H. Hardin, <a href="/A199894/b199894.txt">Table of n, a(n) for n = 1..200</a> %e A199894 Some solutions for n=6 %e A199894 .-2....1....0...-2....3....3....1....3...-2....0....3...-1....1....2....4....1 %e A199894 ..3....0....4....0....0...-3...-1....0....2...-2....0....0...-1...-3...-2...-1 %e A199894 ..0....0...-4....0...-2....0....1...-4...-1....0....0...-1....4....3....1....4 %e A199894 ..1...-4....3....0....1...-3...-2....2....0....2....0....2...-2....0...-2....0 %e A199894 .-2....0...-3...-2...-2....3....2...-4....1....0....1...-4....2....2....2...-4 %e A199894 ..0....3....0....4....0....0...-1....3....0....0...-4....4...-4...-4...-3....0 %K A199894 nonn %O A199894 1,2 %A A199894 _R. H. Hardin_ Nov 11 2011