This course will consist of two parts with the common theme of analyzing a geometric object via geometric sub-objects which are ‘small’ in an appropriate sense. In the first half of the course, we will begin by describing the work on H. W. Lenstra, Jr., on finding generators for the unit groups of subrings of number fields. Lenstra discovered that from the point of view of computational complex...

In this article we prove a generalization of Selberg’s lemma on the existence of torsion free, finite index subgroups of arithmetic groups. Some of the geometric applications are the resolution a conjecture of Nimershiem and answers to questions of Long–Reid and the author.