The Experts below are selected from a list of 210 Experts worldwide ranked by ideXlab platform
R. D. Knowles - One of the best experts on this subject based on the ideXlab platform.
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Modern transport geography
The Geographical Journal, 1993Co-Authors: Brian Hoyle, R. D. KnowlesAbstract:Transport Geography: An Introduction. Transport and Development: Theoretical Approaches. The Role of Transport in the Development Process. Transport Policy and Control. Transport and Environment. Urban Travel Patterns. Urban Transport Problems and Solutions. Inter-Urban Transport. Rural Areas: The Accessibility Problem. Transport for Tourism and Recreation. Bulk Freight Transport. Multimodal Freight Transport. International Surface Passenger Transport: Prospects and Potential. International Air Transport. Sustainability.
Brian Hoyle - One of the best experts on this subject based on the ideXlab platform.
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Modern transport geography
The Geographical Journal, 1993Co-Authors: Brian Hoyle, R. D. KnowlesAbstract:Transport Geography: An Introduction. Transport and Development: Theoretical Approaches. The Role of Transport in the Development Process. Transport Policy and Control. Transport and Environment. Urban Travel Patterns. Urban Transport Problems and Solutions. Inter-Urban Transport. Rural Areas: The Accessibility Problem. Transport for Tourism and Recreation. Bulk Freight Transport. Multimodal Freight Transport. International Surface Passenger Transport: Prospects and Potential. International Air Transport. Sustainability.
Tomasz Jurdzinski - One of the best experts on this subject based on the ideXlab platform.
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Leftist Grammars and the Chomsky Hierarchy
Theory of Computing Systems, 2007Co-Authors: Tomasz Jurdzinski, Krzysztof LorysAbstract:Leftist grammars are characterized in terms of rules of the form a → ba and cd → d, without distinction between terminals and nonterminals. They were introduced by Motwani et al. [13], where the Accessibility Problem for some general protection system was related to these grammars. This protection system was originally proposed in [4] and [15] in the context of Java virtual worlds. The Accessibility Problem is formulated in the form "Can object p gain (illegal) access to object q by a series of legal moves (as prescribed by the policy)?" The membership Problem for leftist grammar is decidable [13], which implies decidability of the Accessibility Problem for the appropriate protection system. We study relationships between language classes defined by various types of leftist grammars and classes of the Chomsky hierarchy. We show that general leftist grammars can define languages which are not context free, answering in the negative a question from [13]. Moreover, we study some restricted variants of leftist grammars and relate them to regular, deterministic context-free, and context-free languages.
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On complexity of grammars related to the safety Problem
Theoretical Computer Science, 2007Co-Authors: Tomasz JurdzinskiAbstract:AbstractLeftist grammars were introduced by Motwani et al., who established the relationship between the complexity of the Accessibility Problem (or safety Problem) for certain general protection systems and the membership Problem for these grammars. The membership Problem for leftist grammars is decidable. This implies the decidability of the Accessibility Problem. It is shown that the membership Problem for leftist grammars is PSPACE-hard. Therefore, the Accessibility Problem in the appropriate protection systems is PSPACE-hard as well. Furthermore, the PSPACE-hardness result is adapted to a very restricted class of leftist grammars, if the grammar is a part of the input
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On Complexity of Grammars Related to the Safety Problem
Lecture Notes in Computer Science, 2006Co-Authors: Tomasz JurdzinskiAbstract:Leftist grammars were introduced by Motwani et. al., who established the relationship between the complexity of Accessibility Problem (or safety Problem) for certain general protection system and the membership Problem of these grammars. The membership Problem for leftist grammars is decidable. This implies the decidability of the Accessibility Problem. It is shown that the membership Problem for leftist grammars is PSPACE-hard. Therefore, the Accessibility Problem in the appropriate protection systems is PSPACE-hard as well. Furthermore, the PSPACE-hardness result is adopted to very restricted class of leftist grammars, if the grammar is a part of the input.
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ICALP (2) - On complexity of grammars related to the safety Problem
Automata Languages and Programming, 2006Co-Authors: Tomasz JurdzinskiAbstract:Leftist grammars were introduced by Motwani et. al., who established the relationship between the complexity of Accessibility Problem (or safety Problem) for certain general protection system and the membership Problem of these grammars. The membership Problem for leftist grammars is decidable. This implies the decidability of the Accessibility Problem. It is shown that the membership Problem for leftist grammars is PSPACE-hard. Therefore, the Accessibility Problem in the appropriate protection systems is PSPACE-hard as well. Furthermore, the PSPACE-hardness result is adopted to very restricted class of leftist grammars, if the grammar is a part of the input
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ICALP (2) - Leftist Grammars Are Non-primitive Recursive
Automata Languages and Programming, 1Co-Authors: Tomasz JurdzinskiAbstract:Leftist grammars were introduced by Motwani et. al. [7], as a tool to show decidability of the Accessibility Problem in certain general protection systems. It is shown that the membership Problem for languages defined by leftist grammars is non-primitive recursive. Therefore, by the reduction of Motwani et. al., the Accessibility Problem in the appropriate protection systems is non-primitive recursive as well.
Mario Cools - One of the best experts on this subject based on the ideXlab platform.
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Detecting urban road network Accessibility Problems using taxi GPS data
Journal of Transport Geography, 2016Co-Authors: Jianxun Cui, Feng Liu, Davy Janssens, Geert Wets, Mario CoolsAbstract:Abstract Urban population growth and economic development have led to the creation of new communities, jobs and services at places where the existing road network might not cover or efficiently handle traffic. This generates isolated pockets of areas which are difficult to reach through the transport system. To address this Accessibility Problem, we have developed a novel approach to systematically examine the current urban land use and road network conditions as well as to identify poorly connected regions, using GPS data collected from taxis. This method is composed of four major steps. First, city-wide passenger travel demand patterns and travel times are modeled based on GPS trajectories. Upon this model, high density residential regions are then identified, and measures to assess Accessibility of each of these places are developed. Next, the regions with the lowest level of Accessibility among all the residential areas are detected, and finally the detected regions are further examined and specific transport situations are analyzed. By applying the proposed method to the Chinese city of Harbin, we have identified 20 regions that have the lowest level of Accessibility by car among all the identified residential areas. A serious reachability Problem to petrol stations has also been discovered, in which drivers from 92.6% of the residential areas have to travel longer than 30 min to refill their cars. Furthermore, the comparison against a baseline model reveals the capacity of the derived measures in accounting for the actual travel routes under divergent traffic conditions. The experimental results demonstrate the potential and effectiveness of the proposed method in detecting car-based Accessibility Problems, contributing towards the development of urban road networks into a system that has better reachability and more reduced inequity.
Seinosuke Toda - One of the best experts on this subject based on the ideXlab platform.
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some observations on the computational complexity of graph Accessibility Problem
Computing and Combinatorics Conference, 1999Co-Authors: Jun Tarui, Seinosuke TodaAbstract:We investigate the space complexity of the (undirected) graph Accessibility Problem (UGAP for short). We first observe that for a given graph G, the Problem can be solved deterministically in space O(sw(G)2 log2 n), where n denotes the number of nodes and sw(G) denotes the separation-width of G that is an invariant of graphs introduced in this paper. We next observe that for the class of all graphs consisting of only two paths, the Problem still remains to be hard for deterministic log-space under the NC1-reducibility. This result tells us that the Problem is essentially hard for deterministic log-space.
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COCOON - Some observations on the computational complexity of graph Accessibility Problem
1999Co-Authors: Jun Tarui, Seinosuke TodaAbstract:We investigate the space complexity of the (undirected) graph Accessibility Problem (UGAP for short). We first observe that for a given graph G, the Problem can be solved deterministically in space O(sw(G)2 log2 n), where n denotes the number of nodes and sw(G) denotes the separation-width of G that is an invariant of graphs introduced in this paper. We next observe that for the class of all graphs consisting of only two paths, the Problem still remains to be hard for deterministic log-space under the NC1-reducibility. This result tells us that the Problem is essentially hard for deterministic log-space.
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Some Observations on the Computational Complexity of Graph Accessibility Problem (Extended Abstract)
Lecture Notes in Computer Science, 1999Co-Authors: Jun Tarui, Seinosuke TodaAbstract:We investigate the space complexity of the (undirected) graph Accessibility Problem (UGAP for short). We first observe that for a given graph G, the Problem can be solved deterministically in space O(sw(G)2 log2 n), where n denotes the number of nodes and sw(G) denotes the separation-width of G that is an invariant of graphs introduced in this paper. We next observe that for the class of all graphs consisting of only two paths, the Problem still remains to be hard for deterministic log-space under the NC1-reducibility. This result tells us that the Problem is essentially hard for deterministic log-space.
