On branching points in the Gilbert-Steiner problem

The Gilbert–Steiner problem is a generalization of the Steiner tree problem and specific optimal mass transportation, which allows the use additional (branching) point in a transport plan. A specific feature of the problem is that the cost of transporting a mass $m$ along a segment of length $l$ is equal to $l\times m^p$ for a fixed $0<p<1$ and segments may end at points not belonging to the supports of given measures (branching points). Main result of this paper determines all pairs of $(p,d)$ for which the Gilbert–Steiner problem in ${\mathbb{R}}^d$ admits only branching points of degree 3. Namely, it happens if and only if $d=2$ or $p<1\mathrm{/}2$.