The transportation problem can be formalized as the problem of finding the optimal paths to transport a measure µ + onto a measure µ − with the same mass. In contrast with the Monge-Kantorovich formalization, recent approaches model the branched structure of such supply networks by an energy functional whose essential feature is to favor wide roads. Given a flow s in a road or a tube or a wire, the transportation cost per unit length is supposed to be proportional to s α with 0 < α < 1. For the Monge-Kantorovich energy α = 1 so that it is equivalent to have two roads with flow 1/2 or a larger one with flow 1. If instead 0 < α < 1, a road with flow s 1 + s 2 is preferable to two individual roads s 1 and s 2 because (s 1 + s 2 ) α < s α 1 + s α 2 . Thus, this very simple model intuitively leads to branched transportation structures. Such a branched structure is observable in ground transportation networks, in draining and irrigation systems, in electric power supply systems and in natural objects like the blood vessels or the trees. When α > 1 − 1 N such structures can irrigate a whole bounded open set of R N . The aim of this paper is to give a mathematical proof of several structure and regularity properties empirically observed in transportation networks. It is first proven that optimal transportation networks have a tree structure and can be monotonically approximated by finite graphs. An interior regularity result is then proven according to which an optimal network is a finite graph away from the irrigated measure. It is also proven that the branching number of optimal networks has everywhere a universal explicit bound. These results answer questions raised in two recent papers by Xia. M. Bernot UMPA, ENS Lyon, 46, Allée d'Italie, 123 280 M. Bernot et al.
Mathematics Subject Classification (2000)Primary: 49Q10; Secondary: 90B10 · 90B06 · 90B20
List of Symbolsor a fixed subset with finite measure of R, set of fiber indices µ + , µ − positive Borel measures in X with equal mass π a probability measure on X × X or "transference plan" t → χ(ω, t), constant for sufficiently large t, Lipschitz path, fiber indexed by ω (ω, t) → χ(ω, t), traffic plan or pattern, set of fibers |χ| := | | the total mass transported by χ P χ the law of ω → χ(ω) ∈ Lip 1 (X ) T (ω)) final point of a fiber π = (τ, σ ) λ transference plan of χ µ + (χ)(A) := |{ω : χ(ω, 0) ∈ A}|, irrigating measure of χ µ + χ = τ λ, irrigating measure of χ µ − (χ)(A) := |{ω : χ(ω, T χ (ω)) ∈ A}|, measure irrigated by χ µ − χ = σ λ, measure irrigated by χ TP(µ + , µ − ) set of traffic plans χ such that µ − χ = µ − and µ + χ = µ + . TP C set of traffic plans such that (G) f (e) α H 1 (e) Gilbert energy M α (µ + , µ − ) minimal transport cost between µ + and µ − for the Xia functional [ω] t ⊂ branch of the fiber ω at time t in a pattern |[ω] t | measure of the branch containing ω at time t E α (χ) =of a traffic plan χ D restriction of χ to a sub-domain of D(χ ) χ x := {ω : x ∈ χ(ω, R)} path class of x in χ x := χ x abbreviation |x| χ = | χx | multiplicity of χ ...