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 A301836 #6 May 04 2021 15:29:28 %S A301836 4,25,139,773,4299,23909,132971,739525,4112907,22874149,127215787, %T A301836 707517317,3934894923,21884125925,121709722091,676895047237, %U A301836 3764587553931,20936952499621,116441967065899,647598149464325 %N A301836 Number of n X 3 0..1 arrays with every element equal to 0, 1 or 2 horizontally or antidiagonally adjacent elements, with upper left element zero. %C A301836 Column 3 of A301841. %H A301836 R. H. Hardin, <a href="/A301836/b301836.txt">Table of n, a(n) for n = 1..210</a> %F A301836 Empirical: a(n) = 7*a(n-1) -8*a(n-2) for n > 3. %e A301836 Some solutions for n=5 %e A301836 ..0..0..0. .0..1..0. .0..1..1. .0..1..0. .0..1..1. .0..0..1. .0..0..1 %e A301836 ..1..1..0. .1..1..1. .0..1..0. .1..0..0. .0..0..0. .1..0..0. .1..0..1 %e A301836 ..1..1..0. .0..0..1. .0..1..0. .1..1..0. .1..0..1. .1..0..0. .1..0..0 %e A301836 ..0..1..0. .1..0..1. .1..1..1. .0..1..0. .1..0..1. .1..0..0. .1..1..0 %e A301836 ..0..1..1. .1..0..1. .0..1..1. .1..1..1. .1..1..0. .1..0..1. .1..1..0 %Y A301836 Cf. A301841. %K A301836 nonn %O A301836 1,1 %A A301836 _R. H. Hardin_, Mar 27 2018