Planar Manhattan local minimal and critical networks

One-dimensional branching extremals of Lagrangian-type functionals are considered. Such extremals appear as a solutions to the classical Steiner problem on a shortest network, i.e., a connected system of paths that has smallest total length among all the networks spanning a given finite set of terminal points in the plane. In the present paper, the Manhattan-length functional is investigated, with Lagrangian equal to the sum of the absolute values of projections of the velocity vector onto the coordinate axes. Such functionals are useful in problems arising in Electronics, Robotics, chip desing, etc. In this case, in contrast to the case of the Steiner problem, local minimality does not imply extremality (however, each extreme network is locally minimal). A criterion of extremality is presented, which shows that the extrmality with respect to the Manhattan-length functional is a global topological property of networks.