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MAPLE
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with (LinearAlgebra):
a:= proc(n) local h, i, M;
M:= Matrix (2*n, shape=symmetric);
for h in [seq (seq ([i, j+n], j=1..n), i=1..n),
seq ([[i, 1+(i mod n)], [n+i, n+1+(i mod n)]][], i=1..n)]
do M[h[]]:= -1 od;
for i to 2*n do M[i, i]:= -add (M[i, j], j=1..2*n) od;
Determinant (DeleteColumn (DeleteRow (M, 1), 1))
end:
seq (a(n), n=3..20); # Alois P. Heinz, Jul 17 2011
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