Discrete & Computational Geometry

, Volume 19, Issue 1, pp 79-94

Lower Bounds on the Distortion of Embedding Finite Metric Spaces in Graphs

  • Y. RabinovichAffiliated withComputer Science Department, University of Toronto, Toronto, Ontario, Canada M5S 1A1 yuri@cs.toronto.edu
  • , R. RazAffiliated withDepartment of Applied Mathematics, Weizmann Institute of Science, Rehovot, Israel ranraz@wisdom.weizmann.ac.il

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Abstract.

The main question discussed in this paper is how well a finite metric space of size n can be embedded into a graph with certain topological restrictions.

The existing constructions of graph spanners imply that any n -point metric space can be represented by a (weighted) graph with n vertices and n 1 +O(1/r) edges, with distances distorted by at most r . We show that this tradeoff between the number of edges and the distortion cannot be improved, and that it holds in a much more general setting. The main technical lemma claims that the metric space induced by an unweighted graph H of girth g cannot be embedded in a graph G (even if it is weighted) of smaller Euler characteristic, with distortion less than g/4 - 3/2 . In the special case when |V(G)| =|V(H)| and G has strictly less edges than H , an improved bound of g/3 - 1 is shown. In addition, we discuss the case χ(G) < χ(H) - 1 , as well as some interesting higher-dimensional analogues. The proofs employ basic techniques of algebraic topology.