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 A268990 #8 Jan 17 2019 17:20:44 %S A268990 7,35,174,849,4083,19416,91491,427863,1988142,9187653,42256599, %T A268990 193542240,883204143,4017241083,18219040206,82410172617,371879874987, %U A268990 1674499435176,7525052043819,33755742643791,151168877259918,675941817039645 %N A268990 Number of n X 3 binary arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two not more than once. %H A268990 R. H. Hardin, <a href="/A268990/b268990.txt">Table of n, a(n) for n = 1..210</a> %F A268990 Empirical: a(n) = 10*a(n-1) - 31*a(n-2) + 30*a(n-3) - 9*a(n-4). %F A268990 Empirical g.f.: x*(7 - 35*x + 41*x^2 - 16*x^3) / (1 - 5*x + 3*x^2)^2. - _Colin Barker_, Jan 17 2019 %e A268990 Some solutions for n=4: %e A268990 ..1..1..0. .0..0..1. .0..0..1. .0..0..0. .1..1..0. .0..1..0. .1..0..0 %e A268990 ..0..1..0. .1..0..0. .1..0..0. .0..0..1. .0..0..0. .0..0..1. .1..0..0 %e A268990 ..0..0..0. .0..0..1. .1..0..0. .0..0..0. .0..0..1. .1..0..0. .1..1..0 %e A268990 ..1..0..1. .0..0..1. .0..1..1. .0..1..0. .0..0..1. .0..1..1. .0..1..0 %Y A268990 Column 3 of A268995. %K A268990 nonn %O A268990 1,1 %A A268990 _R. H. Hardin_, Feb 17 2016