Reachability Oracles for Directed Transmission Graphs

Abstract: Let $P \subset \mathbb{R}<sup>d$ be a set of $n$ points in $d$ dimensions such that each point $p \in P$ has an associated radius $r_p &gt; 0$. The transmission graph $G$ for $P$ is the directed graph with vertex set $P$ such that there is an edge from $p$ to $q$ if and only if $|pq| \leq r_p$, for any $p, q \in P$. A reachability oracle is a data structure that decides for any two vertices $p, q \in G$ whether $G$ has a path from $p$ to $q$. The quality of the oracle is measured by the space requirement $S(n)$, the query time $Q(n)$, and the preprocessing time. For transmission graphs of one-dimensional point sets, we can construct in $O(n \log n)$ time an oracle with $Q(n) = O(1)$ and $S(n) = O(n)$. For planar point sets, the ratio $\Psi$ between the largest and the smallest associated radius turns out to be an important parameter. We present three data structures whose quality depends on $\Psi$: the first works only for $\Psi &lt; \sqrt{3}$ and achieves $Q(n) = O(1)$ with $S(n) = O(n)$ and preprocessing time $O(n\log n)$; the second data structure gives $Q(n) = O(\Psi³ \sqrt{n})$ and $S(n) = O(\Psi³ n<sup>{3/2})$; the third data structure is randomized with $$Q(n) = O(n^{{2/3}\log^{1/3}} \Psi \log^{2/3} n)$$ and $$S(n) = O(n^{{5/3}\log^{1/3}} \Psi \log^{2/3} n)$$ and answers queries correctly with high probability.