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 A183820 #7 Jul 22 2025 09:12:39 %S A183820 1,813,8538066,1163575148283,2051388696859571241 %N A183820 1/256 the number of (n+1)X(n+1) 0..3 arrays with no 2X2 subblock being a reflection across the shared element pair of any horizontal or vertical neighbor. %C A183820 Diagonal of A183827 %e A183820 Some solutions with upper left block zero for 3X3 %e A183820 ..0..0..1....0..0..3....0..0..1....0..0..0....0..0..3....0..0..1....0..0..3 %e A183820 ..0..0..0....0..0..3....0..0..0....0..0..3....0..0..2....0..0..3....0..0..3 %e A183820 ..1..1..3....2..1..1....0..2..1....2..1..0....2..2..2....0..2..1....1..3..1 %K A183820 nonn %O A183820 1,2 %A A183820 _R. H. Hardin_ Jan 07 2011