1987638
On limit graphs of finite vertex-primitive graphs
The class of all connected vertex-transitive graphs forms a metric space under a natural combinatorially defined metric. In this paper we study graphs which are limit points in this metric space of the subset consisting of all finite graphs that admit a vertex-primitive group of automorphisms. A description of these limit graphs provides a useful description of the possible local structures of generic finite graphs that admit a vertex-primitive automorphism group. We give an analysis of the possible types of these limit graphs, and suggest directions for future research. Some of the analysis relies on the finite simple group classification.
206571122
On Half-Transitive Metacirculant Graphs of Prime-Power Order
In this paper, infinitely many graphs of large valency which are half-transitive (that is vertex- and edge-transitive but not arc-transitive) are constructed, and a complete classification is given of half-transitive metacirculant graphs of order a p-power and valency less than 2p, where p is a prime. In particular, it is shown that, for any odd prime p, integers n?3 and k?2 such that k divides p?1, there are exactly (pn?2?1)/2+pn?3?1 nonisomorphic connected half-transitive metacirculants of order pn and valency 2k.
27555290
Constructing Homogeneous Factorisations of Complete Graphs and Digraphs
Abstract. A homogeneous factorisation of a complete graph Kn is a partition of the edge set that is invariant under a subgroup G of Sn such that G is transitive on the parts of the partition and induces a vertex-transitive automorphism group on the graph corresponding to each part. A product construction is given for such factorisations.
16649809
Relative difference sets fixed by inversion and Cayley graphs
Using graph theoretical technique, we present a construction of a (30, 2, 29, 14)-relative difference set fixed by inversion in the smallest finite simple group--the alternating group A5. To our knowledge this is the first example known of relative difference sets in the finite simple groups with a non-trivial forbidden subgroup. A connection is then established between some relative difference sets fixed by inversion and certain antipodal distance-regular Cayley graphs. With the connection, several families of antipodal distance-regular Cayley graphs which are coverings of complete graphs are presented.
23350440
On Isomorphisms of Connected Cayley Graphs, II
A Cayley graph or digraph Cay(G, S) of a finite groupGis called aCI-graphofGif, for anyT?G, Cay(G, S)?Cay(G, T) if and only ifS?=Tfor some??Aut(G). We study the problem of determining which Cayley graphs and digraphs for a given group are CI-graphs. A finite groupGis called aconnected m-DCI-group(orconnected m-CI-group) if all connected Cayley digraphs (or connected Cayley graphs, respectively) ofGof (out)-valency at most m are CI-graphs. For a groupG, letp(G) be the smallest prime divisor of |G|. It was previously shown that all finite groupsGare connected (p(G)?1)-DCI-groups and all finite groupsGare connected 2(p(G)?1)-CI-groups. In this paper, for every primep, we construct infinitely many finite groupsGsuch thatp(G)=pandGis neither a connectedp-DCI-group nor a connected 2p-CI-group, which provides solutions for several open problems in this area.
10480954
Isomorphisms of Finite Cayley Digraphs of Bounded Valency
LetGbe a finite group,Sa subset ofG\{1}, and let Cay(G,S) denote the Cayley digraph ofGwith respect toS. If, for all subsetsS,TofG\{1} of size at mostm, Cay(G,S)?Cay(G,T) implies thatS?=Tfor some??Aut(G), thenGis called anm-DCI-group. In this paper, we prove that, form?2, allm-DCI-groups are of the formU×V, where (|U|, |V|)=1,Uis abelian andVbelongs to an explicitly determined list of groups. Moreover Sylow subgroups of such groups satisfy some very restrictive conditions.
8850461
Schema-guided wrapper maintenance for web-data extraction
Extracting data from Web pages using wrappers is a fundamental problem arising in a large variety of applications of vast practical interests. There are two main issues relevant to Web-data extraction, namely wrapper generation and wrapper maintenance. In this paper, we propose a novel schema-guided approach to the problem of automatic wrapper maintenance. It is based on the observation that despite various page changes, many important features of the pages are preserved, such as syntactic patterns, annotations, and hyperlinks of the extracted data items. Our approach uses these preserved features to identify the locations of the desired values in the changed pages, and repair wrappers correspondingly by inducing semantic blocks from the HTML tree. Our intensive experiments on real Web sites show that the proposed approach can effectively maintain wrappers to extract desired data with high accuracies.
14325657
An approach to reducing the labeling cost of Markov random fields within an infinite label space
This work proposes a novel idea, called SOIL, for reducing the computational complexity of the maximum a posteriori optimization problem using Markov random fields (MRFs). The local characteristics of MRFs are employed so that the searching in a virtually infinite label space is confined in a small finite space. Globally, the number of labels allowed is as many as the number of image sites while locally the optimal label is sampled from a space consisting of the labels assigned to the four-neighbor plus a random one. Neither the prior knowledge about the number of classes nor the estimation phase of the class number is required in this work. The proposed method is applied to the problem of texture segmentation and the result is compared with those obtained from conventional methods.
9529744
Describing and Utilizing Constraints to Answer Queries in Data-Integration Systems
In data-integration systems, information sources often have various constraints such as “all houses stored at a source have a unique address.” These constraints are very useful to compute answers to queries. In this paper we study how to describe these constraints, so that they can be utilized in query processing and optimization. We consider the local-as-view approach to data integration (under the open-world assumption), in which source contents and user queries are formulated on predefined global predicates. In this approach, source contents and constraints often exist before the global predicates are designed. We discuss two different levels of describing constraints: local constraints are defined sources, while global constraints are on global predicates. We formally define two types of global constraints, namely general global constraints and source-derived global constraints. We present the advantages of having these constraints, and discuss open problems that need more research investigations.
19930573
Learning Pathological Characteristics from User's Relevance Feedback for Content-Based Mammogram Retrieval
Content-based image retrieval (CBIR) has been proposed to address the problem of image retrieval from medical image databases. Relevance feedback, explaining the user's query concept, can be used to bridge the semantic gap and improve the performance of CBIR systems. This paper proposes a learning method for relevance feedback, which utilizes probabilistic model to generalize the 2-class problem and provide an estimate of probability of class membership. To build the probabilistic model, support vector machine (SVM) is applied to classify the mammograms, and then scale them to the probability of class membership. Experimental results show that the proposed learning method can effectively improve the average precision rate from 40% to 62% through five iterations of relevance feedback rounds
24632947
Pixelwise fusion for optimizing SNR in multiple-plate computed radiography imaging
The computed radiography (CR) technique, also known as the storage phosphor imaging technique, has evolved to be a major candidate for large-scale implementation of digital radiography during the past decade, In order to obtain a reasonable spatial resolution, the storage phosphor plate used is generally limited in thickness. This leads to X-rays being only partially absorbed by the detector. Useful information may be contained in the X-rays transmitted through the detector. Multiple-plate imaging techniques may be used to capture and utilize the X-rays more efficiently. In this paper, an image fusion method, based on the Rayleigh principle and the Karhunen-Loeve (K-L) transform, is presented for optimizing the signal-to-noise ratio (SNR) of the fused image on a pixel-by-pixel basis. Because the multiple-plate images contain the same structural information, the signal components of the images are highly correlated with one another. Thus, the K-L transform is applied to decompose each of the multiple-plate images into an eigen image (the estimated signal) and a residual image (the estimated noise). An average representation entropy measure is maximized for selecting the number of eigen components to be included in the signal estimation. An experimental study, using an anthropomorphic chest phantom, is presented to illustrate pixelwise fusion of multiple-plate images. Experimental results show that the SNR of the fused image was improved by 12-48%, depending upon the anatomical regions of interest in the image.