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 A227381 #7 Jul 23 2025 05:36:24 %S A227381 2,7,54,1104,61127,8988949,3657501287 %N A227381 Number of n X n 0,1 arrays indicating 2X2 subblocks of some larger (n+1)X(n+1) binary array having a sum of one, with rows and columns of the latter in lexicographically nondecreasing order. %C A227381 Diagonal of A227385 %e A227381 Some solutions for n=4 %e A227381 ..0..0..1..0....0..0..0..0....0..1..0..0....0..0..1..0....0..0..1..0 %e A227381 ..1..0..0..1....0..1..0..0....1..0..0..1....0..1..0..1....1..0..0..0 %e A227381 ..0..0..1..0....1..0..0..0....0..1..1..0....0..0..1..1....0..0..0..1 %e A227381 ..0..0..0..1....0..0..0..0....0..0..0..0....0..0..1..1....0..1..1..0 %K A227381 nonn %O A227381 1,1 %A A227381 _R. H. Hardin_ Jul 09 2013