Sparse Hop Spanners for Unit Disk Graphs
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A unit disk graph G on a given set of points P in the plane is a geometric graph where an edge exists between two points p,q ∈ P if and only if |pq| ≤ 1. A subgraph G' of G is a k-hop spanner if and only if for every edge pq∈ G, the topological shortest path between p,q in G' has at most k edges. We obtain the following results for unit disk graphs. (i) Every n-vertex unit disk graph has a 5-hop spanner with at most 5.5n edges. We analyze the family of spanners constructed by Biniaz (WADS 2019) and improve the upper bound on the number of edges from 9n to 5.5n. (ii) Using a new construction, we show that every n-vertex unit disk graph has a 3-hop spanner with at most 11n edges. (iii) Every n-vertex unit disk graph has a 2-hop spanner with O(n^3/2) edges. This is the first construction of a 2-hop spanner with a subquadratic number of edges. (iv) For every sufficiently large n, there exists a set P of n points such that every plane hop spanner on P has hop stretch factor at least 4. Previously, no lower bound greater than 2 was known. (v) For every point set on a circle, there exists a plane 4-hop spanner. As such, this provides a tight bound for points on a circle. (vi) The maximum degree of k-hop spanners cannot be bounded above by a function of k.
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