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 A304004 #6 Apr 26 2021 14:24:00 %S A304004 1,4,7,13,26,49,99,194,387,773,1538,3081,6147,12298,24587,49157,98330, %T A304004 196609,393251,786450,1572883,3145797,6291442,12583033,25165827, %U A304004 50331738,100663451,201326533,402653578,805306225,1610613155,3221225698 %N A304004 Number of n X 2 0..1 arrays with every element unequal to 0, 2, 3 or 5 king-move adjacent elements, with upper left element zero. %C A304004 Column 2 of A304010. %H A304004 R. H. Hardin, <a href="/A304004/b304004.txt">Table of n, a(n) for n = 1..210</a> %F A304004 Empirical: a(n) = a(n-1) +3*a(n-2) -4*a(n-4) for n > 5. %e A304004 Some solutions for n=5 %e A304004 ..0..0. .0..1. .0..1. .0..1. .0..1. .0..1. .0..1. .0..0. .0..1. .0..1 %e A304004 ..1..1. .0..1. .1..0. .0..1. .1..0. .1..0. .1..0. .0..0. .1..0. .0..1 %e A304004 ..1..1. .0..1. .0..1. .1..0. .0..1. .1..0. .0..1. .0..0. .1..0. .1..0 %e A304004 ..1..1. .0..1. .0..1. .1..0. .0..1. .1..0. .1..0. .0..0. .0..1. .0..1 %e A304004 ..0..0. .0..1. .0..1. .1..0. .1..0. .0..1. .0..1. .0..0. .0..1. .0..1 %Y A304004 Cf. A304010. %K A304004 nonn %O A304004 1,2 %A A304004 _R. H. Hardin_, May 04 2018