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 A206981 #17 Jun 19 2023 15:59:59 %S A206981 4,16,36,100,256,676,1764,4624,12100,31684,82944,217156,568516, %T A206981 1488400,3896676,10201636,26708224,69923044,183060900,479259664, %U A206981 1254718084,3284894596,8599965696,22515002500,58945041796,154320122896,404015326884 %N A206981 Number of n X 2 0..1 arrays avoiding the patterns 0 1 0 or 1 0 1 in any row, column, diagonal or antidiagonal. %H A206981 R. H. Hardin, <a href="/A206981/b206981.txt">Table of n, a(n) for n = 1..210</a> %F A206981 Empirical: a(n) = 2*a(n-1) + 2*a(n-2) - a(n-3). G.f.: -4*x*(-1-2*x+x^2) / ( (1+x)*(x^2-3*x+1) ). %F A206981 Empirical: a(n) = (A080097(n)+1)*4. - _Martin Ettl_, Nov 13 2012 %e A206981 Some solutions for n=4 %e A206981 ..1..1....0..0....0..0....0..0....1..1....0..1....1..1....0..1....1..0....0..0 %e A206981 ..0..1....0..1....1..0....1..0....1..1....1..1....1..1....1..0....0..1....1..1 %e A206981 ..0..0....1..1....1..1....1..0....1..1....1..0....1..1....1..0....0..1....1..1 %e A206981 ..1..0....1..1....0..1....1..0....1..1....1..0....0..0....1..0....1..0....0..0 %Y A206981 Column 2 of A206987. %Y A206981 Cf. A080097. %K A206981 nonn %O A206981 1,1 %A A206981 _R. H. Hardin_, Feb 14 2012