17462310
Bipartite covering graphs
Abstract In this paper we study when a bipartite graph is a covering of a non-bipartite graph. We give a characterization of all bipartite coverings in terms of factoring the covering map through the canonical double covering. We also consider regular bipartite coverings described in terms of voltage assignments. We give an algebraic characterization of such coverings involving the subgroup generated by voltages on closed walks of even length. This allows us to count the number of regular bipartite coverings for orders which are twice a prime.
29011378
The achromatic number of the union of cycles
The achromatic number of a graph G is the largest number of colors which can be assigned to the vertices of G so that adjacent vertices get different colors and each pair of distinct colors appears on the ends of some edge. We show that the achromatic number of the disjoint union of k cycles of length l1, l2 ...., lk is equal to the achromatic number of the cycle of length p = Σi=1k li for any k ≤ √p/2, and that the achromatic number of the disjoint union of k triangles (resp. quadrangles) is equal to the achromatic number of the cycle of length 3k (resp. 4k) for any positive integer k.
26190362
Enumerating branched orientable surface coverings over a non-orientable surface
The isomorphism classes of several types of graph coverings of a graph have been enumerated by many authors [M. Hofmeister, Graph covering projections arising from finite vector space over finite fields, Discrete Math. 143 (1995) 87-97; S. Hong, J.H. Kwak, J.Lee, Regular graph coverings whose covering transformation groups have the isomorphism extention property, Discrete Math. 148 (1996) 85-105; J.H. Kwak, J.H. Chun, J. Lee, Enumeration of regular graph coverings having finite abelian covering transformation groups, SIAM J. Discrete Math. 11 (1998) 273-285; J.H. Kwak, J. Lee, Isomorphism classes of graph bundles, Canad. J. Math. XLII (1990) 747-761; J.H. Kwak, J. Lee, Enumeration of connected graph coverings, J. Graph Theory 23 (1996) 105-109]. Recently, Kwak et al [Balanced regular coverings of a signed graph and regular branched orientable surface coverings over a non-orientable surface, Discrete Math. 275 (2004) 177-193] enumerated the isomorphism classes of balanced regular coverings of a signed graph, as a continuation of an enumeration work done by Archdeacon et al [Bipartite covering graphs, Discrete Math. 214 (2000) 51-63] the isomorphism classes of branched orientable regular surface coverings of a non-orientable surface having a finite abelian covering transformation group. In this paper, we enumerate the isomorphism classes of connected balanced (regular or irregular) coverings of a signed graph and those of unbranched orientable coverings of a non-orientable surface, as an answer of the question raised by Liskovets [Reductive enumeration under mutually orthogonal group actions, Acta-Appl. Math. 52 (1998) 91-120]. As a consequence of these two results, we also enumerate the isomorphism classes of branched orientable surface coverings of a non-orientable surface. Acta-Appl. Math. 52 (1998) 91-120]. As a consequence of these two results, we also enumerate the isomorphism classes of branched orientable surface coverings of a non-orientable surface.
2796328
Adaptive beamforming with multiple-beam constraints in the presence of coherent jammers
Abstract This paper deals with the problem of adaptive beamforming using a uniform linear array to provide multiple-beam constraints and suppress coherent jammers simultaneously. By constructing a steering matrix with each column vector corresponding to the steering vector of a selective beam and a constraint vector with each entry equal to the gain of a selective beam, we can achieve an adaptive array beamformer with multiple-beam constraints. For coping with the performance degradation due to the presence of coherent jammers, a matrix reconstruction scheme in conjunction with an iterative algorithm is presented to alleviate the coherent jamming effect. Simulation examples for illustration and comparison are also given.
17811625
A Graph-Based Approach for Modeling and Indexing Video Data
In this work, we propose new graph-based data model and indexing to organize and manage video data. To consider spatial and temporal characteristics of video, we introduce a new graph-based data model called spatio-temporal region graph (STRG). Unlike existing graph-based data structures which provide only spatial features, the proposed STRG further provides temporal features, which represent temporal relationships among spatial objects. The STRG is decomposed into its subgraphs object graphs (OGs) and background graphs (BGs). In addition, a new distance measure, called extended graph edit distance (EGED), is introduced in metric space for matching and indexing. Based on clustering and EGED, we propose a new indexing method STRG-index, which is faster and more accurate. We compare the STRG-Index with the M-tree, which is a popular tree-based indexing method for multimedia data. The STRG-index outperforms the M-tree in terms of cost and speed
6731652
Nonmonotonic causal theories
The nonmonotonic causal logic defined in this paper can be used to represent properties of actions, including actions with conditional and indirect effects, nondeterministic actions, and concurrently executed actions. It has been applied to several challenge problems in the theory of commonsense knowledge. We study the relationship between this formalism and other work on nonmonotonic reasoning and knowledge representation, and discuss its implementation, called the Causal Calculator.
17722126
Architectural primitives for a scalable shared memory multiprocessor
Since large memory latencies are not uncommon for a large scale multiprocessor, researchers have investigated memory models to relax memory access ordering. This paper presents a new memory model, and discusses implementation issues in a cache-based environment. These issues mot ivate a set of architectural primitives with which software can implement desired memory consistency. For efficient cache management, we propose a cache protocol that allows read-initiated actions for coherence maintenance. For efficient synchronization, we present a cache-based locking scheme that implements queued busy-wait ing using cache lines. The scalability of the proposed schemes is explored through analytical modeling and simulation studies.
9094168
Microscopic examination of TCP flows over transatlantic links
Much of the recent research and development in the area of high-speed TCP is focused on the steady state behavior of TCP flows. However, our experience with the first research only transatlantic 2.5 Gbps Lambda link clearly demonstrates the need to focus on the initial stages of TCP. The work we present here examines the behavior of TCP flows at microscopic level over high-bandwidth long delay networks. This examination has led us to study the influence of the minute properties of the underlying network on bursty protocols such as TCP at these very high speeds combined with high latency. In this paper we briefly describe the requirements for such an extreme network environment to support high-speed TCP flows. We also present results collected using transatlantic links at iGrid2002 where we tuned various host parameters and used modified TCP stacks.