  This chapter is a collection of techniques, warnings, facts and ideas that
are sometimes regarded as theoretical curiosities in high-energy physics but
have important consequences in condensed matter physics. In particular, we
describe theories that have the property of having finite but undetermined
radiative corrections that also happen to describe topological semi-metallic
phases in condensed matter. In the process, we describe typical methods in
high-energy physics that illustrate the working principles to describe a given
phase of matter and its response to external fields.
