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 A297219 #8 Feb 26 2019 11:52:46 %S A297219 3,8,24,68,196,564,1620,4660,13396,38516,110740,318388,915412,2631924, %T A297219 7567124,21756468,62552660,179847028,517083540,1486682292,4274404564, %U A297219 12289467892,35333815316,101589305140,292082437972,839774919796 %N A297219 Number of n X 3 0..1 arrays with every 1 horizontally or antidiagonally adjacent to 1 neighboring 1s. %H A297219 R. H. Hardin, <a href="/A297219/b297219.txt">Table of n, a(n) for n = 1..210</a> %F A297219 Empirical: a(n) = a(n-1) + 4*a(n-2) + 4*a(n-3). %F A297219 Empirical g.f.: x*(3 + 5*x + 4*x^2) / (1 - x - 4*x^2 - 4*x^3). - _Colin Barker_, Feb 26 2019 %e A297219 Some solutions for n=7: %e A297219 ..0..0..0. .0..1..1. .1..1..0. .0..0..0. .0..1..0. .1..1..0. .1..1..0 %e A297219 ..1..1..0. .0..0..1. .0..0..0. .0..1..1. .1..0..1. .0..0..0. .0..0..0 %e A297219 ..0..0..0. .0..1..0. .0..1..0. .0..0..0. .0..1..0. .0..1..1. .1..1..0 %e A297219 ..0..1..0. .0..1..1. .1..0..0. .1..1..0. .0..0..0. .0..0..0. .0..0..0 %e A297219 ..1..0..0. .0..0..0. .1..1..0. .0..1..1. .0..1..1. .0..1..0. .0..1..1 %e A297219 ..0..1..0. .0..0..0. .0..0..0. .0..0..0. .0..0..0. .1..0..0. .0..0..1 %e A297219 ..1..0..0. .0..0..0. .0..0..0. .0..1..1. .1..1..0. .0..1..1. .0..1..0 %Y A297219 Column 3 of A297224. %K A297219 nonn %O A297219 1,1 %A A297219 _R. H. Hardin_, Dec 27 2017