Steiner Transitive-Closure Spanners of d-Dimensional Posets

Given a directed graph G = (V,E) and an integer k ≥ 1, a k-transitive-closure-spanner (k-TCspanner) of G is a directed graph H = (V,EH) that has (1) the same transitive-closure as G and (2) diameter at most k. In some applications, the shortcut paths added to the graph in order to obtain small diameter can use Steiner vertices, that is, vertices not in the original graph G. The resulting spanner is called a Steiner transitive-closure spanner (Steiner TC-spanner). Motivated by applications to property reconstruction and access control hierarchies, we concentrate on Steiner TC-spanners of directed acyclic graphs or, equivalently, partially ordered sets. In these applications, the goal is to find a sparsest Steiner k-TC-spanner of a poset G for a given k and G. The focus of this paper is the relationship between the dimension of a poset and the size of its sparsest Steiner TCspanner. The dimension of a posetG is the smallest d such thatG can be embedded into a d-dimensional directed hypergrid via an order-preserving embedding. We present a nearly tight lower bound on the size of Steiner 2-TC-spanners of d-dimensional directed hypergrids. It implies better lower bounds on the complexity of local reconstructors of monotone functions and functions with low Lipschitz constant. The proof of the lower bound constructs a dual solution to a linear programming relaxation of the Steiner 2-TC-spanner problem. We also show that one can efficiently construct a Steiner 2-TC-spanner, of size matching the lower bound, for any low-dimensional poset. Finally, we present a lower bound on the size of Steiner k-TC-spanners of d-dimensional posets that shows that the best-known construction, due to De Santis et al., cannot be improved significantly. ∗Pennsylvania State University, USA. {berman, sofya, grigory}@cse.psu.edu. S.R. and G.Y. are supported by NSF / CCF CAREER award 0845701. G.Y. is also supported by University Graduate Fellowship and College of Engineering Fellowship. †Massachusetts Institute of Technology, USA. [email protected] ‡Georgia Institute of Technology, USA. [email protected]. Supported in part by NSF award CCR-0829672 and NSF award 1019343 to the Computing Research Association for the Computing Innovation Fellowship Program. §IBM Almaden Research Center, USA. [email protected]. ar X iv :1 01 1. 61 00 v1 [ cs .D S] 2 8 N ov 2 01 0