Sparse Hop Spanners for Unit Disk Graphs Adrian Dumitrescu Department of Computer Science, University of Wisconsin–Milwaukee, WI, USA
[email protected] Anirban Ghosh School of Computing, University of North Florida, Jacksonville, FL, USA
[email protected] Csaba D. Tóth Department of Mathematics, California State University Northridge, Los Angeles, CA, USA Department of Computer Science, Tufts University, Medford, MA, USA
[email protected] Abstract 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 G0 of G is a k-hop spanner if and only if for every edge pq ∈ G, the topological shortest path between p, q in G0 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 (2020) 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 log n) edges. This is the first nontrivial construction of 2-hop spanners. IV For every sufficiently large n, there exists a set P of n points on a circle, such that every plane hop spanner on P has hop stretch factor at least 4.