1,720,958 research outputs found
A time and space efficient simulation algorithm
No full textA number of algorithms for computing the simulation preorder are available. The best simulation algorithms are those by Gentilini, Piazza and Poli- criti (GPP) — subsequently corrected by van Glabbeek and Ploeger — and by Ranzato and Tapparo (RT). Let Σ denote the state space, -> the transition relation and Psim the partition of Σ induced by simulation equivalence. The algorithm GPP attains an optimal space bound of O(|Psim|^2 + |Σ| log |Psim|) — where optimal means that the space complexity is of the same order as the size of the output of the algorithm — while it runs in O(|Psim|^2|->|) time. The algorithm RT has the best time bound O(|Psim||->|) while it takes O(|Psim||Σ| log |Σ|) space. We present here a new time and space Efficient Simulation Algorithm (ESA) that runs in O(|Psim||->|) time and O(|Psim|^2 log |Σ| + |Σ| log |Psim|) space. Thus, ESA reaches in practice the optimal space bound of GPP while preserving the best time bound of RT. Analogously to GPP and RT, ESA is a symbolic algorithm, meaning that it maintains and iteratively refines a partition of the state space together with a preorder relation over this partition, while ESA achieves the above complexity bounds by following Hopcroft’s “process the smaller half” principle when updating some crucial data structures involving the blocks of the state partition
Generalizing the Paige-Tarjan algorithm by abstract interpretation
Full text linkThe Paige and Tarjan algorithm (PT) for computing the coarsest refinement of a state partition which is a bisimulation on some Kripke structure is well known. It is also well known in model checking that bisimulation is equivalent to strong preservation of CTL or, equivalently, of Hennessy-Milner logic. Drawing on these observations, we analyze the basic steps of the PT algorithm from an abstract interpretation perspective, which allows us to reason on strong preservation in the context of arbitrary (temporal) languages and of generic abstract models, possibly different from standard state partitions, specified by abstract interpretation. This leads us to design a generalized Paige-Tarjan algorithm, called GPT, for computing the minimal refinement of an abstract interpretation-based model that strongly preserves some given language. It turns out that PT is a straight instance of GPT on the domain of state partitions for the case of strong preservation of Hennessy-Milner logic. We provide a number of examples showing that GPT is of general use. We first show how a well-known efficient algorithm for computing stuttering equivalence can be viewed as a simple instance of GPT. We then instantiate GPT in order to design a new efficient algorithm for computing simulation equivalence that is competitive with the best available algorithms. Finally, we show how GPT allows to deal with strong preservation of new languages by providing an efficient algorithm that computes the coarsest refinement of a given partition that strongly preserves a language generated by the reachability operator
An efficient simulation algorithm based on abstract interpretation
No full textA number of algorithms for computing the simulation preorder and equivalence are available. Let Σ denote the state space, → the transition relation and Psim the partition of Σ induced by simulation equivalence. The algorithms by Henzinger, Henzinger, Kopke and by Bloom and Paige run in O(|Σ||→|)-time and, as far as time complexity is concerned, they are the best available algorithms. However, these algorithms have the drawback of a space complexity that is more than quadratic in the size of the state space Σ. The algorithm by Gentilini, Piazza, Policriti — subsequently corrected by van Glabbeek and Ploeger — appears to provide the best compromise between time and space complexity. Gentilini et al.’s algorithm runs in O(|Psim|^2|→|)-time while the space complexity is in O(|Psim|^2+|Σ|log|Psim|). We present here a new efficient simulation algorithm that is obtained as a modification of Henzinger et al.’s algorithm and whose correctness is based on some techniques used in applications of abstract interpretation to model checking. Our algorithm runs in O(|Psim||→|)-time and O(|Psim||Σ|log|Σ|)-space. Thus, this algorithm improves the best known time bound while retaining an acceptable space complexity that is in general less than quadratic in the size of the state space |Σ|. An experimental evaluation showed good comparative results with respect to Henzinger, Henzinger and Kopke’s algorithm
Saving space in a time efficient simulation algorithm
Full text linkA number of algorithms for computing the simulation preorder on Kripke structures and on labelled transition systems are available. Among them, the algorithm by Ranzato and Tapparo [2007] has the best time complexity,while the algorithm by Gentilini et al. [2003] – successively corrected by van Glabbeek and Ploeger [2008] – has the best space complexity. Both space and time complexities are critical issues in a simulation algorithm, in particular memory requirements are crucial in the context of model checking when dealing with large state spaces. Here, we propose a new simulation algorithm that is obtained as a space saving modification of the time efficient algorithm by Ranzato and Tapparo: a symbolic representation of sets is embedded in this algorithm so that any set of states manipulated by the algorithm can be efficiently stored as a set of blocks of a suitable state partition. It turns out that this novel simulation algorithm has a space complexity comparable with Gentilini et al.'s algorithm while improving on Gentilini et al.'s time bound
The Subgraph Similarity Problem
Full text linkSimilarity is a well known weakening of bisimilarity where one system is required to simulate the other and vice versa. It has been shown that the subgraph bisimilarity problem, a variation of the subgraph isomorphism problem where isomorphism is weakened to bisimilarity, is NP-complete. We show that the subgraph similarity problem and some related variations thereof still remain NP-complete
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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