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 A269209 #8 Jan 20 2019 08:56:41 %S A269209 24,768,18048,361728,6712704,118872576,2039727744,34214296320, %T A269209 564168820608,9179147971584,147751571802240,2357407757843712, %U A269209 37337046775523712,587670598254177792,9200252201653201536 %N A269209 Number of n X 3 0..3 arrays with some element plus some horizontally, diagonally or antidiagonally adjacent neighbor totalling three exactly once. %H A269209 R. H. Hardin, <a href="/A269209/b269209.txt">Table of n, a(n) for n = 1..210</a> %F A269209 Empirical: a(n) = 30*a(n-1) - 237*a(n-2) + 180*a(n-3) - 36*a(n-4) for n>5. %F A269209 Empirical g.f.: 24*x*(1 + 2*x + 29*x^2 - 84*x^3 + 36*x^4) / (1 - 15*x + 6*x^2)^2. - _Colin Barker_, Jan 20 2019 %e A269209 Some solutions for n=3: %e A269209 ..0..0..0. .2..0..0. .3..2..3. .1..3..1. .2..0..0. .2..3..3. .2..3..3 %e A269209 ..0..0..2. .2..2..2. .2..2..3. .1..1..1. .1..2..2. .1..3..3. .3..1..3 %e A269209 ..1..2..2. .0..3..2. .3..3..0. .3..3..0. .3..3..2. .2..0..2. .0..1..0 %Y A269209 Column 3 of A269214. %K A269209 nonn %O A269209 1,1 %A A269209 _R. H. Hardin_, Feb 20 2016