In these lectures we present some useful techniques to study quantitative
properties of solutions of elliptic PDEs. Our aim is to outline a proof of a
recent result on propagation of smallness. The ideas are also useful in the
study of the zero sets of eigenfunctions of Laplace-Beltrami operator and we
discuss the connection. Some basic facts about second order elliptic PDEs in
divergent form are collected in the Appendix at the end of the notes.Comment: Lecture notes for Graduate Summer School in Park City (PCMI), July
201