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 A213692 #5 Nov 16 2021 21:46:44 %S A213692 28,1320,208072,109820000,193946752094,1146407046198040, %T A213692 22678299640600736258,1501445596428593204920908, %U A213692 332684482425964910985336666019,246706401290583205383638262307956104 %N A213692 Half the number of (n+1) X (n+1) symmetric 0..3 arrays with no 2 X 2 subblock summing to 6. %C A213692 Column 3 of A213697. %e A213692 Some solutions for n=4 %e A213692 ..0..2..1..0..1....0..0..1..2..0....0..0..3..0..1....2..1..1..2..1 %e A213692 ..2..0..2..2..1....0..3..3..1..1....0..1..3..1..2....1..1..0..2..0 %e A213692 ..1..2..0..3..1....1..3..0..1..2....3..3..2..1..0....1..0..3..0..0 %e A213692 ..0..2..3..1..2....2..1..1..1..1....0..1..1..3..3....2..2..0..2..1 %e A213692 ..1..1..1..2..0....0..1..2..1..0....1..2..0..3..2....1..0..0..1..1 %Y A213692 Cf. A213697. %K A213692 nonn %O A213692 1,1 %A A213692 _R. H. Hardin_ Jun 18 2012