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 A239333 #7 Feb 18 2018 08:40:25 %S A239333 2,5,12,28,66,156,368,868,2048,4832,11400,26896,63456,149712,353216, %T A239333 833344,1966112,4638656,10944000,25820224,60917760,143723520, %U A239333 339087488,800010496,1887468032,4453111040,10506243072,24787422208,58481066496 %N A239333 Number of n X 1 0..3 arrays with no element equal to one plus the sum of elements to its left or three plus the sum of elements above it, modulo 4. %C A239333 Column 1 of A239340. %H A239333 R. H. Hardin, <a href="/A239333/b239333.txt">Table of n, a(n) for n = 1..210</a> %F A239333 Empirical: a(n) = 2*a(n-1) +2*a(n-3). %F A239333 Empirical g.f.: (2 + x + 2*x^2) / (1 - 2*x - 2*x^3). - _Colin Barker_, Feb 18 2018 %e A239333 Some solutions for n=5: %e A239333 ..0....2....2....2....0....0....0....0....2....2....2....2....2....0....2....2 %e A239333 ..2....0....2....0....2....0....2....0....0....0....0....2....2....2....0....3 %e A239333 ..2....2....2....3....2....2....3....0....2....2....3....0....0....0....3....2 %e A239333 ..0....0....0....2....2....2....2....0....2....2....3....0....2....0....2....3 %e A239333 ..2....2....2....0....2....0....0....0....3....0....2....2....3....2....3....2 %K A239333 nonn %O A239333 1,1 %A A239333 _R. H. Hardin_, Mar 16 2014