| Popis: |
An $(\alpha,\beta)$ spanner of an edge weighted graph $G=(V,E)$ is a subgraph $H$ of $G$ such that for every pair of vertices $u$ and $v$, $d_{H}(u,v) \le \alpha \cdot d_G(u,v) + \beta W$, where $d_G(u,v)$ is the shortest path length from $u$ to $v$ in $G$; we consider two settings: in one setting $W = W_G(u,v),$ the maximum edge weight in a shortest path from $u$ to $v$ in $G$, and in the other setting $W=W_{max},$ the maximum edge weight of $G$. If $\alpha>1$ and $\beta=0$, then $H$ is called a multiplicative $\alpha$-spanner. If $\alpha=1$, then $H$ is called an additive +$\beta W$ spanner. While multiplicative spanners are very well studied in the literature, spanners that are both additive and lightweight have been introduced more recently [Ahmed et al., WG 2021]. Here the lightness is the ratio of the spanner weight to the weight of a minimum spanning tree of $G$. In this paper, we examine the widely known subsetwise setting when the distance conditions need to hold only among the pairs of a given subset $S$. We generalize the concept of lightness to subset-lightness using a Steiner tree and provide polynomial-time algorithms to compute subsetwise additive $+\epsilon W(\cdot, \cdot)$ spanner and $+(4+\epsilon) W(\cdot, \cdot)$ spanner with $O_\epsilon(|S|)$ and $O_\epsilon(|V_H|^{1/3} |S|^{1/3})$ subset-lightness, respectively, where $\epsilon$ is an arbitrary positive constant. We next examine a multi-level version of spanners that often arises in network visualization and modeling the quality of service requirements in communication networks. The goal here is to compute a nested sequence of spanners with the minimum total edge weight. We provide an $e$-approximation algorithm to compute multi-level spanners assuming that an oracle is given to compute single-level spanners, improving a previously known 4-approximation [Ahmed et al., IWOCA 2023]. |
