THE STEINER PROBLEM IN THE PLANE OR IN PLANE MINIMAL NETS
Alexander Ivanov, Alexey Avgustinovich Tuzhilin · Mathematics of the USSR-Sbornik · 1993
The famous Steiner problem in the Euclidean plane, which is that of investigating minimal nets spanning fixed finite subsets M of points in the plane, is solved when M is extremal, i.e. when M lies on the boundary of its convex hull, and the nets are nondegenerate, i.e. have no vertices of degree 2.