1,721,045 research outputs found
On semidefinite lift-and-project of combinatorial optimization problems
No full textFinding the stability and the chromatic number of a graph are two among the fundamental problems in combinatorial optimization. Given a graph, the first calls for a stable set of maximum cardinality, i.e. a subset of vertices such that no two are adjacent; the latter asks for a partition of the nodes into the minimum number of stable sets (i.e. colors).
Both the stable set and graph coloring problems are well-known to be NP-hard, hence no
polynomial time algorithm to solve them exactly is expected to exists unless P=NP. Thus, the study of strong relaxations of these two problems is a well-researched topic. In particular, the Lov\'asz theta function provides at the same time a good upper bound on the stability number of a graph and a lower bound on the chromatic number of its complement. It can be computed in polynomial time by solving a semidefinite program, which in addition turns often out to be fairly tractable in practice. As a consequence, achieves a hard-to-beat trade-off between computational effort and strength of the bound. Hierarchies of relaxations to strengthen both towards the stability and chromatic number have been documented, but in general such improvements come at a heavy computational burden with off-the-shelf SDP algorithms and require highly specialized methods to be addressed.
In the last decades, Lift-and-Project methods have gained a lot of attention, being able to generate strong relaxations for combinatorial optimization problems. In particular, starting from any linear relaxation \lov\ and \sch's Lift-and-Project framework generates a semidefinite relaxation. Its application to the fractional stable set polytope showed its potential, producing bounds stronger than but in general they come at a nontrivial computational cost.
In this thesis we introduce a new semidefinite relaxation for the stable set problem obtained by the lifting of a more compact linear formulation than the classical one. Then, we characterize some classes of valid inequalities for the stable set polytope which are implied by our proposal. We then discuss how to face the computational burden arising from these semidefinite programs by the employment of a general purpose solver for SDPs.
Despite Lift-and-Project applications have been widely studied on the Stable Set problem, to the best of our knowledge none on the Graph Coloring problem have been presented. We investigate its employment in this problem, showing that the resulting SDP can yield bounds above the fractional chromatic number, a remarkable threshold not so straightforward to cross with semidefinite programming.
Although interior-point methods achieve good accuracy in reasonable time for small and medium size SDPs, their scalability to large instances is often compromised by memory requirements.
On the other hand, Alternating Direction Methods of Multipliers currently represent the most popular first-order alternatives, being suited to scale to much larger semidefinite programs. This of course at some cost in accuracy, that should be correctly addressed when bounding the optimal solution of some combinatorial optimization problem. In this work we focus on an ADMM designed for SDPs in standard form and extend it to deal with inequalities. Moreover, we report different methods to compute a valid bound on the optimal value of the SDP starting from a medium accuracy solution and we discuss the employments of these methodologies within ADMMs
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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