Janka Chlebíková

Approximation hardness of dominating set problems in bounded degree graphs

We study approximation hardness of the Minimum Dominating Set problem and its variants in undirected and directed graphs. Using a similar result obtained by Trevisan for Minimum Set Cover we prove the first explicit approximation lower bounds for various kinds of domination problems (connected, total, independent) in bounded degree graphs. Asymptotically, for degree bound approaching infinity, these bounds almost match the known upper bounds. The results are applied to improve the lower bounds for other related problems such as Maximum Induced Matching and Maximum Leaf Spanning Tree.

Complexity of approximating bounded variants of optimization problems

We study low degree graph problems such as Maximum Independent Set and Minimum Vertex Cover. The goal is to improve approximation lower bounds for them and for a number of related problems like Max-B-Set Packing, Min-B-Set Cover, and Max-B-Dimensional Matching, B≥3. We prove, for example, that it is NP-hard to achieve an approximation factor of 95/94 for Max-3-DM, and a factor of 48/47 for Max-4-DM. In both cases the hardness result applies even to instances with exactly two occurrences of each element.

Approximation Hardness of the Steiner Tree Problem on Graphs

Steiner tree problem in weighted graphs seeks a minimum weight subtree containing a given subset of the vertices (terminals). We show that it is NP-hard to approximate the Steiner tree problem within 96/95. Our inapproximability results are stated in parametric way and can be further improved just providing gadgets and/or expanders with better parameters. The reduction is from Hastads inapproximability result for maximum satisfiability of linear equations modulo 2 with three unknowns per equation. This was first used for the Steiner tree problem by Thimm whose approach was the main starting point for our results.

The Steiner tree problem on graphs: Inapproximability results

The Steiner tree problem on weighted graphs seeks a minimum weight subtree containing a given subset of the vertices (terminals). We show that it is NP-hard to approximate the Steiner tree problem within a factor 96/95. Our inapproximability results are stated in a parametric way, and explicit hardness factors would be improved automatically by providing gadgets and/or expanders with better parameters.

Inapproximability Results for Bounded Variants of Optimization Problems

We study small degree graph problems such as Maximum Independent Set and Minimum Node Cover and improve approximation lower bounds for them and for a number of related problems, like Max- B -Set Packing, Min- B -Set Cover, Max-Matching in B-uniform 2-regular hypergraphs. For example, we prove NP-hardness factor of \(\frac{95}{94}\) for Max-3DM, and factor of \(\frac{48}{47}\) for Max-4DM; in both cases the hardness result applies even to instances with exactly two occurrences of each element.

Approximating the maximally balanced connected partition problem in graphs

The approximability of the following optimization problem is investigated: Given a connected graph G = (V, E), find the maximally balanced connected partition for G, i.e. a partition (V1, V2) of V into disjoint sets V1 and V2 such that both subgraphs of G induced by V1 and V2 are connected, and maximize an objective function “balance”, B(V1, V2) = min(¦V1¦, ¦V2¦). We prove that for any ϵ > 0 it is NP-hard (even for bipartite graphs) to approximate the maximum balance of the connected partition for G = (V, E) with an absolute error guarantee of ¦V¦1 − e. On the other hand, we give a polynomial-time approximation algorithm that solves the problem within 43 even when vertices of G are weighted. The variation of the problem is equivalent to the Maximally Balanced Spanning Tree Problem studied by Galbiati, Maffioli and Morzenti (1995). Our simple polynomial-time algorithm approximates the solution of that problem within 1.072.

Approximation hardness of edge dominating set problems

We provide the first interesting explicit lower bounds on efficient approximability for two closely related optimization problems in graphs, MINIMUM EDGE DOMINATING SET and MINIMUM MAXIMAL MATCHING. We show that it is NP-hard to approximate the solution of both problems to within any constant factor smaller than

Approximation Hardness of Dominating Set Problems

{\frac{7}{6}}