Kac polynomials and Lie algebras associated to quivers and curves

A survey of the theory of Kac polynomials for quivers and for curves. In particular, we describe the representation-theoretic meaning of Kac polynomials in terms of Hall algebras, and the geometric meaning of Kac polynomials in relation to the geometry of moduli spaces of representations of quivers or vector bundles on smooth projective curves. We end with some heuristics concerning a family of infinite-dimensional $\mathbb{Z}^2$-graded Lie algebras attached to curves of a fixed genus (over a finite field), whose 'Cartan datum' encodes the dimension of the spaces of absolutely cuspidal functions.