  The group is interesting as the first example of split rank 2 semisimple
group, all the irreducible unitary representations of which are known. We make
a precise realization of the discrete series representations (in Section 2) by
using the Orbit Method and Geometric Quantization, a computation of their
traces (Section 3) and an exact formula for the noncommutative Poisson
summation and endoscopy of for this group (in Section 4).
