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 A187458 #7 Jul 22 2025 10:45:54 %S A187458 6,26,144,1272,15192,286392,7648032,314011868,18986004296, %T A187458 1725663296600,237096836118080,48071903199959400,15027587384189632928, %U A187458 6819730931931151055960,4852231403635188536789728 %N A187458 Number of (n+1)X(n+1) 0..1 arrays with all the 2X2 subblocks nonsingular and the array of 2X2 subblock determinants symmetric. %C A187458 Column 1 of A187462 %H A187458 R. H. Hardin, <a href="/A187458/b187458.txt">Table of n, a(n) for n = 1..18</a> %e A187458 Some solutions for 4X4 %e A187458 ..1..1..0..1....1..0..1..1....0..1..0..1....1..0..1..1....1..0..1..0 %e A187458 ..1..0..1..0....0..1..0..1....1..0..1..0....0..1..0..1....1..1..1..1 %e A187458 ..0..1..0..1....1..0..1..1....1..1..0..1....1..0..1..1....1..0..1..0 %e A187458 ..1..0..1..1....0..1..0..1....1..0..1..1....1..1..1..0....0..1..1..1 %K A187458 nonn %O A187458 1,1 %A A187458 _R. H. Hardin_ Mar 10 2011