The Geometry of the Minimal Resultant Locus

Abstract

Let K be a complete, algebraically closed, nonarchimedean valued field, and let f(z) be a rational function in K(z) of degree d at least 2. We show there is a natural way to assign non-negative integer weights w_f(P) to points of the Berkovich projective line over K, in such a way that the sum over all points is d-1. When f(z) has bad reduction, the set of points with nonzero weight forms a distributed analogue of the single point which occurs when f(z) has potential good reduction. Using this, we characterize the Minimal Resultant Locus of f(z) in dynamical and moduli-theoretic terms: dynamically, it is the barycenter of the weight-measure attached to f(z); moduli-theoretically, it is the closure of the set of type II points where f(z) has semi-stable reduction in the sense of Geometric Invariant Theory.