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 A269284 #8 Jan 21 2019 06:12:42 %S A269284 60,2124,62748,1698732,43674876,1085203980,26317946844,626778812268, %T A269284 14718495557052,341767357185996,7863372461151900,179542238849355564, %U A269284 4073026776304945788,91888997272241919372,2063133077083679521116 %N A269284 Number of n X 3 0..3 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling three no more than once. %H A269284 R. H. Hardin, <a href="/A269284/b269284.txt">Table of n, a(n) for n = 1..210</a> %F A269284 Empirical: a(n) = 42*a(n-1) - 441*a(n-2). %F A269284 Conjectures from _Colin Barker_, Jan 21 2019: (Start) %F A269284 G.f.: 12*x*(5 - 33*x) / (1 - 21*x)^2. %F A269284 a(n) = 4 * 3^n * 7^(n-2) * (24*n+11). %F A269284 (End) %e A269284 Some solutions for n=3: %e A269284 ..2..0..0. .0..1..3. .0..2..3. .2..0..2. .2..2..3. .3..0..1. .3..1..0 %e A269284 ..0..2..0. .3..1..2. .3..3..3. .1..0..0. .0..1..0. .1..3..2. .1..1..0 %e A269284 ..1..1..0. .1..0..0. .2..3..3. .0..1..3. .1..0..2. .2..0..1. .3..0..2 %Y A269284 Column 3 of A269289. %K A269284 nonn %O A269284 1,1 %A A269284 _R. H. Hardin_, Feb 21 2016