The beta invariant of an arrangement
A is, by definition, the Euler characteristic of complement of
A in complex projective space.
The beta invariant of a flat
F is the beta invariant of the
restriction of
A to
F.
i1 : A = typeA(3)
o1 = {x - x , x - x , x - x , x - x , x - x , x - x }
1 2 1 3 1 4 2 3 2 4 3 4
o1 : Hyperplane Arrangement
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i2 : euler A -- for a real arrangement, equals number of bounded chambers
o2 = 2
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