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 A187421 #7 Jul 22 2025 10:43:44 %S A187421 609,6512,46764,602024,5254072,56472058,533853394,5585054046, %T A187421 54998694426,562340568708,5598043072500,56738877828678, %U A187421 568753193822092,5743694780294918,57708886492197048,581740972471740850 %N A187421 Number of (n+1)X4 0..2 arrays with every 2X2 subblock determinant equal to exactly one or two horizontal and vertical neighbor 2X2 subblock determinants. %C A187421 Column 3 of A187425 %H A187421 R. H. Hardin, <a href="/A187421/b187421.txt">Table of n, a(n) for n = 1..200</a> %e A187421 Some solutions for 3X4 %e A187421 ..2..1..0..1....0..0..0..1....2..2..0..2....0..2..2..2....1..0..2..0 %e A187421 ..2..2..2..1....0..0..1..1....1..2..0..0....0..1..1..2....2..1..2..1 %e A187421 ..0..1..2..0....2..0..2..1....0..2..0..0....1..1..0..2....1..1..0..1 %K A187421 nonn %O A187421 1,1 %A A187421 _R. H. Hardin_ Mar 09 2011