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 A259420 #7 Jun 02 2025 11:50:57 %S A259420 13,243,12953,1990133,873754597,1101527291223,3975872829274865, %T A259420 41147590225188806309,1220087829652769318601383, %U A259420 103689338398592714772850061461,25251868912730969096083539641411367 %N A259420 Number of (n+1)X(n+1) 0..1 arrays with each 2X2 subblock having clockwise pattern 0000 0001 0011 or 0111. %C A259420 Diagonal of A259427 %H A259420 R. H. Hardin, <a href="/A259420/b259420.txt">Table of n, a(n) for n = 1..14</a> %e A259420 Some solutions for n=3 %e A259420 ..1..1..1..1....0..0..0..0....0..0..1..1....0..0..1..1....1..1..1..0 %e A259420 ..0..0..0..0....1..0..0..1....0..0..0..0....0..0..1..0....1..0..1..0 %e A259420 ..0..0..1..0....0..0..1..1....1..1..1..0....0..0..1..1....1..0..1..1 %e A259420 ..1..1..1..1....0..0..0..1....1..0..1..0....1..0..0..0....1..1..1..0 %Y A259420 Cf. A259427 %K A259420 nonn %O A259420 1,1 %A A259420 _R. H. Hardin_, Jun 26 2015