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 A213693 #5 May 08 2023 15:06:28 %S A213693 56,5470,2322599,4286833981,34389632236877,1199143411128185584, %T A213693 181744271406372884215557,119728593804039164010819562504 %N A213693 Half the number of (n+1) X (n+1) symmetric 0..4 arrays with no 2 X 2 subblock summing to 8. %C A213693 Column 4 of A213697. %e A213693 Some solutions for n=4 %e A213693 ..4..4..4..2..1....3..1..2..4..0....1..2..4..4..0....1..0..0..2..4 %e A213693 ..4..4..1..0..1....1..1..2..4..3....2..1..3..4..1....0..1..1..4..3 %e A213693 ..4..1..1..0..4....2..2..2..1..1....4..3..2..1..0....0..1..2..2..4 %e A213693 ..2..0..0..0..3....4..4..1..1..2....4..4..1..0..0....2..4..2..1..4 %e A213693 ..1..1..4..3..1....0..3..1..2..0....0..1..0..0..4....4..3..4..4..2 %Y A213693 Cf. A213697. %K A213693 nonn %O A213693 1,1 %A A213693 _R. H. Hardin_, Jun 18 2012