One-dimensional Gromov minimal filling
  The present paper opens a new branch in the theory of variational problems with branching extremals, the investigation of one-dimensional minimal fillings of finite pseudo-metric spaces. On the one hand, this problem is a one-dimensional version of a generalization of Gromov's minimal fillings problem to the case of stratified manifolds (the filling in our case is a weighted graph). On the other hand, this problem is interesting in itself and also can be considered as a generalization of another classical problem, namely, the Steiner problem on the construction of a shortest network joining a given set of terminals. Besides the statement of the problem, we discuss several properties of the minimal fillings, describe minimal fillings of additive spaces, and state several conjectures. We also include some announcements concerning the very recent results obtained in our group, including a formula calculating the weight of the minimal filling for an arbitrary finite pseudo-metric space and the concept of pseudo-additive space which generalizes the classical concept of additive space. We hope that the theory of one-dimensional minimal fillings refreshes the interest in the Steiner problem and gives an opportunity to solve several long standing problems, such as the calculation of the Steiner ratio, in particular the verification of the Gilbert--Pollack conjecture on the Steiner ratio of the Euclidean plane. 
A.O.Ivanov and A.A.Tuzhilin
math.MG