The uniformization
theorem of Poincaré and Koebe tells us that every smooth connected algebraic
curve X over the complex numbers (or any Riemann surface) has as its
universal covering space either the complex projective line PC1, the complex
numbers C, or the complex upper half plane H = {z ∈ C s.t. Imz > 0}.
When the universal covering space is the upper half plane H, we can regard
the fundamental group π1(X) as a subgroup of SL2(R) acting as covering
transformations via linear fractional transformation. We shall focus on the case
π1(X) ⊂SL2(Z).