Convex minorant trees associated with Brownian paths and the continuum limit of the minimum spanning tree

TL;DR

This paper constructs the scaling limit of the minimum spanning tree of a complete graph using convex minorants of Brownian motion, revealing new structural insights and connections to continuum random trees.

Contribution

It introduces the Brownian parabolic tree as a new representation of the MST limit and develops the theory of convex minorant trees associated with Brownian-like paths.

Findings

Hausdorff dimension of the Brownian parabolic tree is almost surely 3

Constructs a metric multiplicative coalescent linking different graph scales

Shows convex minorant trees include Brownian continuum random trees

Abstract

We give an explicit construction of the scaling limit of the minimum spanning tree of the complete graph. The limit object is described using a recursive construction involving the convex minorants of a Brownian motion with parabolic drift (and countably many i.i.d. uniform random variables); we call it the Brownian parabolic tree. Aside from the new representation, this point of view has multiple consequences. For instance, it permits us to prove that its Hausdorff dimension is almost surely 3. It also intrinsically contains information related to some underlying dynamics: one notable by-product is the construction of a standard metric multiplicative coalescent which couples the scaling limits of random graphs at different points of the critical window in terms of the same simple building blocks. The above results actually fit in a more general framework. They result from the introduction of a new family of continuum random trees associated with functions via their convex minorants, that we call convex minorant trees. We initiate the study of these structures in the case of Brownian-like paths. In passing, we prove that the convex minorant tree of a Brownian excursion is a Brownian continuum ranndom tree, and that it provides a coupling between the Aldous--Pitman fragmentation of the Brownian continuum random tree and its representation by Bertoin.