scispace - formally typeset
Search or ask a question

Showing papers on "Network theory published in 2000"


Journal ArticleDOI
TL;DR: In this paper, the authors use network constructions to show that the minimum L0-norm problem is NP-hard, or one that complexity theory suggests is impossible for efficient algorithms to solve exactly.
Abstract: Two-dimensional (2-D) phase unwrapping, that is, deducing unambiguous phase values from a 2-D array of values known only modulo 2pi, is a key step in interpreting data acquired with synthetic aperture radar interferometry. Noting the recent network formulation of the phase unwrapping problem, we apply here some well-established ideas of network theory to formalize the problem, analyze its complexity, and derive algorithms for its solution. It has been suggested that the objective of phase unwrapping should be to minimize the total number of places where unwrapped and wrapped phase gradients differ. Here we use network constructions to show that this so-called minimum L0-norm problem is NP-hard, or one that complexity theory suggests is impossible for efficient algorithms to solve exactly. Therefore we must instead find approximate solutions; we present two new algorithms for doing so. The first uses the network ideas of shortest paths and spanning trees to improve on the Goldstein et al. residue-cut algorithm [Radio Sci. 23, 713 (1988)]. Our improved algorithm is very fast, provides complete coverage, and allows user-defined weights. With our second algorithm, we extend the ideas of linear network flow problems to the nonlinear L0 case. This algorithm yields excellent approximations to the minimum L0 norm. Using interferometric data, we demonstrate that our algorithms are highly competitive with other existing algorithms in speed and accuracy, outperforming them in the cases presented here.

473 citations


Journal ArticleDOI
TL;DR: In this article, it is suggested that one way of advancing the policy network approach is to apply collective action theory and explicitly regard different empirical appearances of network concepts as expressions of collective action.
Abstract: An important contribution to the policy sciences, and to interorganizational research in particular, has been the introduction of the so-called policy network approach. Despite the fact that this approach has produced a multitude of concepts, it still lacks a theoretical scaffold. In this article it is argued that simply to refer to something called “network theory” is an unsatisfactory solution. It is suggested that one way of advancing the policy network approach is to apply collective action theory and explicitly regard different empirical appearances of network concepts as expressions of collective action. Six tentative building blocks of such a theory are suggested. It is further argued that the policy network approach would benefit from incorporation into a broader analytical framework such as the Institutional Analysis and Development framework. Finally, it is concluded that such an incorporation would advance our ability to understand the processes of policymaking and thus to fulfill one of the old commitments of policy analysis, namely to contribute to the refinement of policymaking processes in society.

213 citations


Journal ArticleDOI
TL;DR: New measures of centrality that summarize the contact structure of social networks are proposed that consider all the possible paths, do not require intensive computer calculations, and can be used to compare networks of different sizes because they are independent of the size of the networks.

96 citations


Book
01 Jan 2000
TL;DR: Fundamentals of network theory the transmission and reflection coefficients elements of passive network synthesis filter approximation and realization circuit optimization impedance transformation broadband matching networks compatible impedances real-frequency solutions of the broadband matching problems diplexer and multiplexer.
Abstract: Fundamentals of network theory the transmission and reflection coefficients elements of passive network synthesis filter approximation and realization circuit optimization impedance transformation broadband matching networks compatible impedances real-frequency solutions of the broadband matching problems diplexer and multiplexer.

11 citations



Book ChapterDOI
01 Jan 2000
TL;DR: In this paper, the authors deal with basic graph theoretical concepts and then focus on special topics which are of relevance to theoretical and practical problems in OR, such as Connectivity, Menger's Theorem, the Splitting Lemma, and Factors.
Abstract: This chapter deals with basic graph theoretical concepts and then focuses on special topics which are — in their applications — of relevance to theoretical and practical problems in OR. Therefore, this chapter is structured as follows: 1. Basics of Graph Theory. 2. Connectivity, Menger’s Theorem, the Splitting Lemma, and Factors. 3. Eulerian Graphs and Covering Walks, Cycle Decompositions and Cycle Covers. 4. Hamiltonian Cycles and Vertex-Covering Walks. 5. Elements of Matching Theory. 6. The Chinese Postman Problem, The Traveling Salesman Problem, and Related Problems. 7. Elements of Network Theory.

7 citations


Proceedings ArticleDOI
21 Aug 2000
TL;DR: This work introduces a graph theoretical approach for an information delivery problem in ad-hoc networks and shows how coloring algorithms of graphs are applicable in channel assignment algorithms in cellular systems.
Abstract: The demand for communication services is rapidly increasing, because the mobile communication service is synonymous with an ideal communication style realizing communication anytime, anywhere and with anyone. There exist various problems to which graph and network theory is applicable in mobile communication services. For example, it is well known that coloring algorithms of graphs are applicable in channel assignment algorithms in cellular systems. We introduce a graph theoretical approach for an information delivery problem in ad-hoc networks.

2 citations


Proceedings ArticleDOI
12 Nov 2000
TL;DR: In this paper, a basic problem in printed circuit board (PCB) assembly planning for the case of Tchebyshev robot arm motion in which it will be used for the other problems is considered.
Abstract: This paper considers a basic problem in printed circuit board (PCB) assembly planning for the case of Tchebyshev robot arm motion in which it will be used for the other problems. In this problem, the assembly sequence is determined when feeder rack travel with infinite speed and the board is fixed. The problem is analyzed and converted into a shortest path problem of a corresponding interval graph that is proved to be NP-complete. Therefore, a heuristic procedure is developed based on network theory and interval graph. This heuristic is tested and found to give the closed optimal solution.

1 citations


Journal ArticleDOI
S. Hamao1
TL;DR: The fact that analyzing the feedback circuit using the Superposition theorem is inaccurate is unveiled, because the network theory, which is based on Energy Conservation Law, is applied to a circuit that has no relationship with it.
Abstract: It may have been believed that the majority of problems in network theory have been solved, but it is not necessarily true. In this paper the Instantaneous Propagation Problem, presented by Debart is shown and the Anti-Causality Problem is introduced as contradicted examples of the Reciprocity Theorem and Anti-Reciprocity Theorem, respectively. Since the circuit of interest is a feedback circuit, the input signal circulates the feedback loop infinitely. We unveil the fact that analyzing the feedback circuit using the Superposition theorem is inaccurate, because the network theory, which is based on Energy Conservation Law, is applied to a circuit that has no relationship with it.

1 citations


01 Jan 2000
TL;DR: A graph theoretical approach for an inforination delivering problem in ad-hoc networks is introduced and it is shown that coloring algorithms of graphs are applicable in channel assignment algorithms in cellular systems.
Abstract: The demand for communication services is rapidly increasing, because the mobile communication service is synonymy of an ideal communication style realizing communication in anytime, anywhere and with anyone. There exist various problems to which graph & network theory is applicable in mobile communication services. For cxanipie, it is wcll known that coloring algorithms of graphs are applicable in channel assignment algorithms in cellular systems. In this paper, we introduce a graph theoretical approach for an inforination delivering problem in ad-hoc networks.