Pascal Schweitzer

Approximation Algorithms for Capacitated Minimum Forest Problems in Wireless Sensor Networks with a Mobile Sink

To deploy a wireless sensor network for the purpose of large-scale monitoring, in this paper, we propose a heterogeneous and hierarchical wireless sensor network architecture. The architecture consists of sensor nodes, gateway nodes, and mobile sinks. The sensors transmit their sensing data to the gateway nodes for temporary storage through multihop relays, while the mobile sinks travel along predetermined trajectories to collect data from nearby gateway nodes. Under this paradigm of data gathering, we formulate a novel constrained optimization problem, namely, the capacitated minimum forest (CMF) problem, for the decision version of which we first show NP-completeness. We then devise approximation algorithms and provide upper bounds for their approximation ratios. We finally evaluate the performance of the proposed algorithms through experimental simulation. In our experiments, the approximation ratio delivered by the proposed algorithms is always less than 2. In the case of arbitrary gateway capacities, this contrasts our theoretical results which show that the approximation ratio is at most linear in the number of gateways. Our experiments thus indicate that for realistic inputs, our worst case analysis of the approximation ratio is very conservative. The proposed algorithms are the first approximation algorithms for the CMF problem, and our techniques may be applicable to other constrained optimization problems beyond wireless sensor networks.

Isomorphism for graphs of bounded feedback vertex set number

This paper presents an

A linear time algorithm for testing maximal 1-planarity of graphs with a rotation system

{\mathcal O}(n^2)

Paging and list update under bijective analysis

algorithm for deciding isomorphism of graphs that have bounded feedback vertex set number. This number is defined as the minimum number of vertex deletions required to obtain a forest. Our result implies that Graph Isomorphism is fixed-parameter tractable with respect to the feedback vertex set number. Central to the algorithm is a new technique consisting of an application of reduction rules that produce an isomorphism-invariant outcome, interleaved with the creation of increasingly large partial isomorphisms.

Isomorphism Testing for Graphs of Bounded Rank Width

A 1-planar graph is a graph that can be embedded in the plane with at most one crossing per edge. It is known that testing 1-planarity of a graph is NP-complete. In this paper, we consider maximal 1-planar graphs. A graph G is maximal 1-planar if addition of any edge destroys 1-planarity of G. We first study combinatorial properties of maximal 1-planar embeddings. In particular, we show that in a maximal 1-planar embedding, the graph induced by the non-crossing edges is spanning and biconnected. Using the properties, we show that the problem of testing maximal 1-planarity of a graph G can be solved in linear time, if a rotation system @F (i.e., the circular ordering of edges for each vertex) is given. We also prove that there is at most one maximal 1-planar embedding @x of G that is consistent with the given rotation system @F. Our algorithm also produces such an embedding in linear time, if it exists.

Homomorphism–homogeneous graphs

It has long been known that for the paging problem in its standard form, competitive analysis cannot adequately distinguish algorithms based on their performance: there exists a vast class of algorithms that achieve the same competitive ratio, ranging from extremely naive and inefficient strategies (such as Flush-When-Full), to strategies of excellent performance in practice (such as Least-Recently-Used and some of its variants). A similar situation arises in the list update problem: in particular, under the cost formulation studied by Martínez and Roura [2000] and Munro [2000] every list update algorithm has, asymptotically, the same competitive ratio. Several refinements of competitive analysis, as well as alternative performance measures have been introduced in the literature, with varying degrees of success in narrowing this disconnect between theoretical analysis and empirical evaluation. In this article, we study these two fundamental online problems under the framework of bijective analysis [Angelopoulos et al. 2007, 2008]. This is an intuitive technique that is based on pairwise comparison of the costs incurred by two algorithms on sets of request sequences of the same size. Coupled with a well-established model of locality of reference due to Albers et al. [2005], we show that Least-Recently-Used and Move-to-Front are the unique optimal algorithms for paging and list update, respectively. Prior to this work, only measures based on average-cost analysis have separated LRU and MTF from all other algorithms. Given that bijective analysis is a fairly stringent measure (and also subsumes average-cost analysis), we prove that in a strong sense LRU and MTF stand out as the best (deterministic) algorithms.

Testing maximal 1-planarity of graphs with a rotation system in linear time

We give an algorithm that, for every fixed k, decides isomorphism of graphs of rank width at most k in polynomial time. As the rank width of a graph is bounded in terms of its clique width, we also obtain a polynomial time isomorphism test for graph classes of bounded clique width.

Towards an Isomorphism Dichotomy for Hereditary Graph Classes

We answer two open questions posed by Cameron and Nesetril concerning homomorphism–homogeneous graphs. In particular we show, by giving a characterization of these graphs, that extendability to monomorphism or to homomorphism leads to the same class of graphs when defining homomorphism–homogeneity. Further, we show that there are homomorphism–homogeneous graphs that do not contain the Rado graph as a spanning subgraph answering the second open question. We also treat the case of homomorphism–homogeneous graphs with loops allowed, showing that the corresponding decision problem is co–NP complete. Finally, we extend the list of considered morphism–types and show that the graphs for which monomorphisms can be extended to epimor-phisms are complements of homomorphism–homogeneous graphs.

Infeasible code detection

A 1-planar graph is a graph that can be embedded in the plane with at most one crossing per edge. It is known that testing 1-planarity of a graph is NP-complete. A 1-planar embedding of a graph G is maximal if no edge can be added without violating the 1-planarity of G. In this paper we show that the problem of testing maximal 1-planarity of a graph G can be solved in linear time, if a rotation system (i.e., the circular ordering of edges for each vertex) is given. We also prove that there is at most one maximal 1-planar embedding of G that preserves the given rotation system, and our algorithm produces such an embedding in linear time, if it exists.

A Linear-Time Algorithm for Testing Outer-1-Planarity

In this paper we resolve the complexity of the isomorphism problem on all but finitely many of the graph classes characterized by two forbidden induced subgraphs. To this end we develop new techniques applicable for the structural and algorithmic analysis of graphs. First, we develop a methodology to show isomorphism completeness of the isomorphism problem on graph classes by providing a general framework unifying various reduction techniques. Second, we generalize the concept of the modular decomposition to colored graphs, allowing for non-standard decompositions. We show that, given a suitable decomposition functor, the graph isomorphism problem reduces to checking isomorphism of colored prime graphs. Third, we extend the techniques of bounded color valence and hypergraph isomorphism on hypergraphs of bounded color class size as follows. We say a colored graph has generalized color valence at most k if, after removing all vertices in color classes of size at most k, for each color class C every vertex has at most k neighbors in C or at most k non-neighbors in C. We show that isomorphism of graphs of bounded generalized color valence can be solved in polynomial time.