We consider minimum-cost spanning trees, both in lattice and Euclidean models, in d dimensions. For the cost of the optimum tree in a box of size L, we show that there is a correction of order L θ , where θ ≤ 0 is a universal d-dependent exponent. There is a similar form for the change in optimum cost under a change in boundary condition. At non-zero temperature T , there is a crossover length ξ ∼ T −ν , such that on length scales larger than ξ, the behavior becomes that of uniform spanning trees. There is a scaling relation θ = −1/ν, and we provide several arguments that show that ν and −1/θ both equal νperc, the correlation length exponent for ordinary percolation in the same dimension d, in all dimensions d ≥ 1. The arguments all rely on the close relation of Kruskal's greedy algorithm for the minimum spanning tree, percolation, and (for some arguments) random resistor networks. The scaling of the entropy and free energy at small non-zero T , and hence of the number of near-optimal solutions, is also discussed. We suggest that the Steiner tree problem is in the same universality class as the minimum spanning tree in all dimensions, as is the traveling salesman problem in two dimensions. Hence all will have the same value of θ = −3/4 in two dimensions.
I. INTRODUCTIONMinimum spanning trees are a problem of combinatorial optimization [1,2]. Suppose we are given an undirected connected graph G, with vertex set V and edge set E, and a cost (or weight, or "length") ℓ ij assigned to each edge ij ∈ E (where i, j ∈ V ). The problem is to find a spanning tree T (i.e. a connected subgraph of G that includes all vertices in V , but whose edges form no cycles; such a tree must have |V | − 1 edges), such that the total cost of the edges in T,is as small as possible. Thus the minimization is over the set T of spanning trees in G.In this paper we are interested in the case in which G is a simply-connected portion Λ of a regular lattice in d ≥ 1 dimensions (with edges connecting nearest-neighbor lattice vertices only; the nearest-neighbor distance is fixed at 1 throughout this paper), including the case when Λ tends to the entire lattice, and the edge costs are independent, identically-distributed random variables, for example ℓ ij uniformly distributed on [0, 1]. We will also consider geometries with periodic boundary conditions, in which Λ has no boundary. The results also apply without significant modification to cases with other distributions, and/or with short-range correlations of the ℓ ij s, and to the Euclidean minimum spanning tree, in which N = |V | points are distributed independently and uniformly (with density 1) in a portion Λ of d-dimensional Euclidean space, and the cost of an edge ij is the Euclidean distance between i and j, for any pair i = j.The motivation for this work is to understand disordered systems at low temperatures better, beginning with those in which quantum-mechanical effects are negligible. Here "disordered" means that the Hamiltonian (or energy as a function of the system configuration) con...