Equivariant cohomology and rings of functions

Abstract

This submission is a PhD dissertation. It constitutes the summary of the author's work concerning the relations between cohomology rings of algebraic varieties and rings of functions on zero schemes and fixed point schemes. It includes the results from the co-authored article arXiv:2212.11836. They are complemented by: an introduction to the theory of group actions on algebraic varieties, with particular focus on vector fields; a historical overview of the field; a few newer results of the author. The fundamental theorem from arXiv:2212.11836 says that if the principal nilpotent has a unique zero, then the zero scheme over the Kostant section is isomorphic to the spectrum of the equivariant cohomology ring, remembering the grading in terms of a $\mathbb{C}$ action. In this thesis, we also tackle the case of a singular variety. As long as it is embedded in a smooth variety with regular action, we are able to study its cohomology as well by means of the zero scheme. In largest generality, this allows us to see geometrically a subring of the cohomology ring. We also show that the cohomology ring of spherical varieties appears as the ring of functions on the zero scheme. Lastly, the K-theory conjecture is studied, with some results attained for GKM spaces.