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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.

A187669 Number of (n+1)X(n+1) 0..2 arrays with each 2X2 subblock nonsingular and the array of 2X2 subblock determinants symmetric about the diagonal and antidiagonal.

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%I A187669 #6 Jul 22 2025 10:51:55
%S A187669 50,550,11282,612204,65533922,18950885038
%N A187669 Number of (n+1)X(n+1) 0..2 arrays with each 2X2 subblock nonsingular and the array of 2X2 subblock determinants symmetric about the diagonal and antidiagonal.
%C A187669 Column 2 of A187670
%e A187669 Some solutions for 4X4
%e A187669 ..1..0..1..2....2..0..2..1....1..1..2..0....2..2..2..2....1..0..1..0
%e A187669 ..2..2..0..2....2..1..0..2....1..2..1..2....2..1..0..2....1..2..0..1
%e A187669 ..1..0..1..2....2..0..1..1....2..1..2..1....2..0..1..2....1..0..2..1
%e A187669 ..0..2..0..2....1..2..0..2....0..2..1..1....1..2..2..2....0..1..0..1
%K A187669 nonn
%O A187669 1,1
%A A187669 _R. H. Hardin_ Mar 12 2011

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