Approximation Algorithms for Independence and Domination on B 1 -VPG and B 1 -EPG Graphs.

A graph $G$ is called B$_k$-VPG (resp., B$_k$-EPG), for some constant $k\geq 0$, if it has a string representation on a grid such that each vertex is an orthogonal path with at most $k$ bends and two vertices are adjacent in $G$ if and only if the corresponding strings intersect (resp., the corresponding strings share at least one grid edge). If two adjacent strings of a B$_k$-VPG graph intersect exactly once, then the graph is called a one-string B$_k$-VPG graph. In this paper, we study the Maximum Independent Set and Minimum Dominating Set problems on B$_1$-VPG and B$_1$-EPG graphs. We first give a simple $O(\log n)$-approximation algorithm for the Maximum Independent Set problem on B$_1$-VPG graphs, improving the previous $O((\log n)^2)$-approximation algorithm of Lahiri et al. (COCOA 2015). Then, we consider the Minimum Dominating Set problem. We give an $O(1)$-approximation algorithm for this problem on one-string B$_1$-VPG graphs, providing the first constant-factor approximation algorithm for this problem. Moreover, we show that the Minimum Dominating Set problem is APX-hard on B$_1$-EPG graphs, ruling out the possibility of a PTAS unless P=NP. Finally, we give constant-factor approximation algorithms for this problem on two non-trivial subclasses of B$_1$-EPG graphs. To our knowledge, these are the first results for the Minimum Dominating Set problem on B$_1$-EPG graphs, partially answering a question posed by Epstein et al. (WADS 2013).