96295
Efficient algorithms for pattern matching on directed acyclic graphs
Recently graph data models have become increasingly popular in many scientific fields. Efficient query processing over such data is critical. Existing works often rely on index structures that store pre-computed transitive relations to achieve efficient graph matching. In this paper, we present a family of stack-based algorithms to handle path and twig pattern queries for directed acyclic graphs (DAGs) in particular. With the worst-case space cost linearly bounded by the number of edges in the graph, our algorithms achieve a quadratic runtime complexity in the average size of the query variable bindings. This is optimal among the navigation-based graph matching algorithms.
28954888
Modeling shape and topology of 3D images of biological specimens
This paper presents an efficient way of representing the geometry and topology of 3D images of biological specimens derived from electron microscopy. A vector quantization algorithm is used in order to select a reduced set of interior points that best approximates the probability density function of the original volumetric data. Object's geometry and topology are obtained from the point set by the use of the alpha shapes theory.
9743987
Complex Visual Activity Recognition Using a Temporally Ordered Database
We propose using a temporally ordered database for complex visual activity recognition. We use a temporal precedence relation together with the assumption of fixed bounded temporal uncertainty of occurrence time of an atomic activity and comparatively large temporal extent of the complex activity. Under these conditions we identify the temporal structure of complex activities as a semiorder and design a database that has semiorder as its data model. A query algebra is then defined for this data model.
8323337
Modeling Functional Data Sources as Relations
In this paper we present a model of functional access to data that, we argue, is suitable for modeling a class of data repositories characterized by functional access, such as web sites. We discuss the problem of modeling such data sources as a set of relations, of determining whether a given query expressed on these relations can be translated into a combination of functions defined by the data sources, and of finding an optimal plan to do so.
14666453
An Interpolated Volume Data Model
Publisher Summary This chapter explores the basic functionality of the volume data model. It considers examples from biology and volumes of measurement from quantum physics. The data model is similar in the two cases, but the operations that are commonly used are quite different. In the second case, for instance, one is often interested in conditions on the behavior of local differential operators, such as zero-flow surfaces, while in the biological case one has a mix of value conditions and geometric condition. With these two application fields, this chapter highlights the generality and flexibility of the model. The system is based on a commercial database, augmented with specialized functions to manipulate the volume data model. The chapter demonstrates various operations specifying queries both using a graphical user interface and entering them directly in structured query language (SQL) augmented with the volume algebra operations.
2635468
A System for Managing Alternate Models in Model-Based Mediation
In [1,3], we have described the problem of model-based mediation (MBM) as an extension of the global-as-view paradigm of information integration. The need for this extension arises in many application domains where the information sources to be integrated not only differ in their export formats, data models, and query capabilities, but have widely different schema with very little overlap in attributes. In scientific applications, the information sources come from different subdisciplines, and despite their poorly overlapping schema, can be integrated because they capture different aspects of the same scientific objects or phenomena, and can be conceptually integrated due to scientific reasons. In the MBM paradigm, a “mediation engineer” consults with domain experts to explicitly model the “glue knowledge” using a set of facts and rules at the mediator. Integrated views are defined in MBM on top of the exported schemas from the information sources together with the glue knowledge source that ties them together. We have successfully applied the MBM technique to develop the KIND mediator or Neuroscience information sources [1]-[4]. To accomplish this, sources in the MBM framework export their conceptual models (CMs), consisting of the logical schema, domain constraints, and object contexts, i.e., formulas that relate their conceptual schema with the global domain knowledge maintained at the mediator. Thus model-based mediation has a hybrid approach to information integration - on the one hand at the mediator integrated views are defined over source CMs and the Knowledge Map using a global-as-view approach; on the other hand, object-contexts of the source are defined as local-as-view.
2250792
Revising TimeGraph-II
TimeGraph-II is a system for efficient point-based temporal reasoning, developed by Schubert, Gerevini, and collaborators. For our purposes, temporal relations are represented in a directed acyclic graph, where vertices represent time points and directed edges are labelled ≤ or <, with no redundant edges. The graph is partitioned into a set of chains, supported by a metagraph data structure. Temporal reasoning can be carried out efficiently: reasoning within a chain takes constant time; reasoning between chains is less efficient, but is determined by a graph (the metagraph) significantly smaller than the original. We suggest two enhancements to TimeGraph-II, both of which lead to improvements in the original approach. First, the original approach does not consider updates to the structure. We show that additions to chains can be accomplished very efficiently, with manageable degradation in reasoning (O(log n) in both cases). Second, we argue that basing the representation on trees rather than chains leads to a strictly more powerful reasoner. More queries can be answered in constant time, and at the same time, for general queries, the size of the metagraph is decreased.
204999323
Acyclic Directed Hypercubes may have Exponential Diameter
Abstract The diameter of a directed graph is the maximum of the lengths of the shortest paths between all pairs of vertices (if such paths exist). We prove that for every n , the edges of the n -dimensional hypercube can be acyclically oriented such that the diameter of the cube is F n + 1 , the ( n + 1)st Fibonacci number.
1412337
Simpler and better approximation algorithms for network design
We give simple and easy-to-analyze randomized approximation algorithms for several well-studied NP-hard network design problems. Our algorithms improve over the previously best known approximation ratios. Our main results are the following.We give a randomized 3.55-approximation algorithm for the connected facility location problem. The algorithm requires three lines to state, one page to analyze, and improves the best-known performance guarantee for the problem.We give a 5.55-approximation algorithm for virtual private network design. Previously, constant-factor approximation algorithms were known only for special cases of this problem.We give a simple constant-factor approximation algorithm for the single-sink buy-at-bulk network design problem. Our performance guarantee improves over what was previously known, and is an order of magnitude improvement over previous combinatorial approximation algorithms for the problem.
35215015
Coarse Grain Preconditioned Conjugate Gradient Solver for Large Sparse Systems
We explore a coarse grain parallel algorithm for the solution of large sparse linear systems with symmetric positive definite matrix, based on a preconditioned conjugate gradient method.
6390859
Updating <=, <-chains
We address the problem of very efficient reasoning and update in ≤, <-chains, where a ≤, <-chain is a directed acyclic graph such that there is a directed path between every pair of vertices, and edges are consistently labelled by ≤ or <. It is easy to show that, subsequent to an O(n) labelling scheme, queries concerning implied ≤ and < relations can be answered in O(1) time. However this scheme does not allow for efficient updates to a chain. We show via a novel encoding of precedence information that updates to chains can be accomplished very efficiently. For edge or vertex addition, updates can be carried out in O(log n) time, with manageable degradation for queries: ≤ queries are answered in O(1) time while < queries require O(log n) time. This result is surprising, in that in the obvious approach to updates O(n) time is required.