lecture notes from the cours given at VIASM school on Number Theory (18-24/06/2018, Hanoi, Vietnam). Submitted.

#### Abstract:

The lecture notes below correspond to the course given by the author in occasion of the VIASM school on Number Theory (18-24 June 2018, Hanoi). We have chosen to omit the proofs that are already presented in details in many references in the literature, although they were explained during the lectures, and we have devoted more space to statements useful in the applications.