14351696
A sufficient condition for a semicomplete multipartite digraph to be Hamiltonian
Abstract A multipartite tournament is an orientation of a complete k -partite graph for some k ⩾ 2. A factor of a digraph D is a collection of vertex disjoint cycles covering all the vertices of D . We show that there is no degree of strong connectivity which together with the existence of a factor will guarantee that a multipartite tournament is Hamiltonian. Our main result is a sufficient condition for a multipartite tournament to be Hamiltonian. We show that this condition is general enough to provide easy proofs of many existing results on paths and cycles in multipartite tournaments. Using this condition, we obtain a best possible lower bound on the length of a longest cycle in any strongly connected multipartite tournament.
4778324
Pushing vertices in digraphs without long induced cycles
Given a digraph D and a subset X of vertices of D, pushing X in D means reversing the orientation of all arcs with exactly one end in X. We continue the study of deciding whether a digraph can be made acyclic using the push operation, focussing on special classes of well-structured digraphs. It is proved that the problem remains NP-complete even when restricted to the class of bipartite digraphs (i.e., oriented bipartite graphs). We characterize, in terms of two forbidden subdigraphs, the chordal digraphs which can be made acyclic using the push operation, and show that there is no similar characterization for the family of chordal bipartite digraphs. A polynomial algorithm, based on 2-SAT, for solving the problem for a subclass of the chordal bipartite digraphs is given. Finally, a characterization in terms of a finite number of forbidden subdigraphs, of the acyclically pushable bipartite permutation digraphs is given.
31025959
A Note on Spanning Local Tournaments in Locally Semicomplete Digraphs
We prove a result which implies that every k-connected locally semicomplete digraph which is not a semicomplete digraph must contain a k-connected spanning local tournament. This improves an earlier result of Guo and partially answers a question of Jackson and Thomassen.
4803217
Pushing the cycles out of multipartite tournaments
Abstract Let D be a digraph and X ⊆ V ( D ). By pushing X we mean reversing each arc of D with exactly one end in X. Klostermeyer proved that it is NP-complete to decide if a given digraph can be made acyclic using the push operation. Here we characterize, in terms of forbidden subdigraphs, the multipartite tournaments which can be made acyclic (resp. ordinary, unidirectional) using the push operation. This implies that the problem of deciding if a given multipartite tournament can be made acyclic (resp. ordinary, unidirectional) using the push operation and, if so, finding a suitable subset of vertices to push, is solvable in polynomial time.
10675163
Maximal and Minimal Vertex-Critical Graphs of Diameter Two
A graph is vertex-critical if deleting any vertex increases its diameter. We construct, for each??5 except?=6, a vertex-critical graph of diameter two on?vertices with at least 12?2?2?3/2+c1?edges, wherec1is some constant. We show that such a graph contains at most 12?2?(2/2)?3/2+c2?edges, wherec2is some constant. We also construct, for each??5 except?=6, a vertex-critical graph of diameter two on?vertices with at most 12 (5??12) edges. We show that such a graph must contain at least 12 (5??29) edges.
3890410
Strongly Connected Spanning Subdigraphs with the Minimum Number of Arcs in Quasi-transitive Digraphs
We consider the problem of finding a strongly connected spanning subdigraph with the minimum number of arcs in a strongly connected digraph. This problem is NP-hard for general digraphs since it generalizes the Hamiltonian cycle problem. We show that the problem is polynomially solvable for quasi-transitive digraphs. We describe the minimum number of arcs in such a spanning subdigraph of a quasi-transitive digraph in terms of the path covering number. Our proofs are based on a number of results (some of which are new and interesting in their own right) on the structure of cycles and paths in quasi-transitive digraphs and in extended semicomplete digraphs. In particular, we give a new characterization of the longest cycle in an extended semicomplete digraph. Finally, we point out that our proofs imply that the MSSS problem is solvable in polynomial time for all digraphs that can be obtained from strong semicomplete digraphs on at least two vertices by replacing each vertex with a digraph belonging to a family of digraphs whose path covering number can be decided in polynomial time.
16266978
Ontology Reconciliation for Service-Oriented Computing
Service-oriented computing (SOC) is viewed as the computing paradigm of the near future, allowing for the dynamic interaction of services provided by distributed business partners. Being a declarative knowledge representation model, ontologies serve as a foundation for SOC. Due to the heterogeneous nature of independently designed ontologies, it is problematic for partners to understand the concepts adopted in ontologies from other sources. In order for partners to achieve seamless collaboration of services, they need to reconcile their ontologies with each other. During the alignment process and the following service interactions, compatibility is an important measurement that has been neglected in most research work. We extend a vector system to encode ontology compatibility. In addition, we present a new model - probabilistic center ontology - for better recording and maintenance of ontology alignment results. Our precise and efficient approach is verified by both theoretic proofs and experimental results
2763612
Information Marginalization on Subgraphs
Real-world data often involves objects that exhibit multiple relationships; for example, ‘papers' and ‘authors' exhibit both paper-author interactions and paper-paper citation relationships. A typical learning problem requires one to make inferences about a subclass of objects (e.g. ‘papers'), while using the remaining objects and relations to provide relevant information. We present a simple, unified mechanism for incorporating information from multiple object types and relations when learning on a targeted subset. In this scheme, all sources of relevant information are marginalized onto the target subclass via random walks. We show that marginalized random walks can be used as a general technique for combining multiple sources of information in relational data. With this approach, we formulate new algorithms for transduction and ranking in relational data, and quantify the performance of new schemes on real world data—achieving good results in many problems.
33920606
Hot-Spot Passage Retrieval in Question Answering
Question Answering has been the recent focus of information retrieval research; many systems just incorporate a search engine as a black box and most effort has been devoted to the question analyzer and the answer identifier. In the context of QA, however, passage provides an ideal medium between the document collection and an exact answer. And passage retrieval is a finer-grain approach than the traditional document retrieval both for the answer identifier and a human reader. In this paper, distinctions are first made between document retrieval and passage retrieval. And the Hot-Spot Passage Retrieval algorithm, which takes into account the measures of blurred BM25, coverage and height, is examined in detail. For evaluation, an isolated test is conducted and the algorithm gains 18.3% better answer redundancy and 4.8% better coverage rate than Okapi's original passage retrieval algorithm.
2023739
Joint video scene segmentation and classification based on hidden Markov model
Video classification and segmentation are fundamental steps for efficient accessing, retrieval and browsing of large amounts of video data. We have developed a scene classification scheme using a hidden Markov model (HMM) based classifier. By utilizing the temporal behaviors of different scene classes, the HMM classifier can effectively classify video segments into one of the pre-defined scene classes. In this paper, we describe two approaches for joint video classification and segmentation based on a HMM, which works by searching for the most likely class transition path utilizing the dynamic programming technique.
392551
Finding Target Language Correspondence for Lexicalized EBMT System
This paper presents a three-phase approach to find the correspondence in Target Language (TL) sentence for a fragment of Source Language (SL) sentence in a lexicalized EBMT system. To be practical, it exploits surface information as much as possible instead of using parsers. Experiments show that, although not so perfect, it is very robust and effective. The three phases are: First, align the sentence pair at word level in order to provide anchors for phrase alignment. Second, based on the aligned anchors, find all the TL fragments which may possibly correspond to a SL fragment. And finally, using a score function, select the best TL fragment as the correspondence of a SL fragment.