The transportation problem can be formalized as the problem of ?nding +
? the optimal paths to transport a measure ? onto a measure ? 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 ?ow ? in a tube or a road or a wire, the transportation cost per unit
length ? is supposed to be proportional to ? with 0