## Institute for Mathematical Sciences

## Preprint ims98-7

** J.J.P. Veerman, M.M. Peixoto, A.C. Rocha, and S. Sutherland**
* On Brillouin Zones*

Abstract: Brillouin zones were introduced by Brillouin in the thirties to describe quantum mechanical properties of crystals, that is, in
a lattice in $\R^n$. They play an important role in solid-state
physics. It was shown by Bieberbach that Brillouin zones tile
the underlying space and that each zone has the same area. We
generalize the notion of Brillouin Zones to apply to an
arbitrary discrete set in a proper metric space, and show that
analogs of Bieberbach's results hold in this context.
We then use these ideas to discuss focusing of geodesics in
orbifolds of constant curvature. In the particular case of the
Riemann surfaces $\H^2/\Gamma (k)$ (k=2,3, or 5), we
explicitly count the number of geodesics of length $t$ that
connect the point $i$ to itself.

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