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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.

A187668 Number of (n+1)X(n+1) 0..1 arrays with each 2X2 subblock nonsingular and the array of 2X2 subblock determinants symmetric about the diagonal and antidiagonal.

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%I A187668 #7 Jul 22 2025 10:51:48
%S A187668 6,24,112,974,9928,197152,4547688,200947508,10553936856,1051895421812,
%T A187668 125339066655024,28078885802576900,7575400864257725704
%N A187668 Number of (n+1)X(n+1) 0..1 arrays with each 2X2 subblock nonsingular and the array of 2X2 subblock determinants symmetric about the diagonal and antidiagonal.
%C A187668 Column 1 of A187670
%e A187668 Some solutions for 4X4
%e A187668 ..1..0..1..0....0..1..1..1....0..1..0..1....1..1..1..0....1..1..1..1
%e A187668 ..1..1..1..1....1..1..0..1....1..1..1..0....0..1..0..1....1..0..1..0
%e A187668 ..1..0..1..0....1..0..1..1....0..1..0..1....1..0..1..0....0..1..0..1
%e A187668 ..0..1..0..1....0..1..1..0....1..1..1..0....1..1..0..1....1..1..1..1
%K A187668 nonn
%O A187668 1,1
%A A187668 _R. H. Hardin_ Mar 12 2011

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