1,720,961 research outputs found
Branching on multi-aggregated variables
Full text linkAbstract. In mixed-integer programming, the branching rule is a key component to a fast convergence of the branch-and-bound algorithm. The most common strategy is to branch on simple disjunctions that split the domain of a single integer variable into two disjoint intervals. Multi-aggregation is a presolving step that replaces variables by an affine linear sum of other variables, thereby reducing the problem size. While this simplification typically improves the performance of MIP solvers, it also restricts the degree of freedom in variable-based branching rules. We present a novel branching scheme that tries to overcome the above drawback by considering general disjunctions defined by multi-aggregated variables in addition to the standard disjunctions based on single vari-ables. This natural idea results in a hybrid between variable- and con-straint-based branching rules. Our implementation within the constraint integer programming framework SCIP incorporates this into a full strong branching rule and reduces the number of branch-and-bound nodes on a general test set of publicly available benchmark instances. For a specific class of problems, we show that the solving time decreases significantly.
Exakte und schnelle Algorithmen für gemischt-ganzzahlige nichtlineare Programmierung
No full textMixed-integer nonlinear programming (MINLP) comprises the broad class of finite-dimensional mathematical optimization problems from mixed-integer linear programming and global optimization. The combination of the two disciplines allows us to construct more accurate models of real-world systems, while at the same time it increases the algorithmic challenges that come with solving them. This thesis presents new methods that improve the numerical reliability and the computational performance of global MINLP solvers. Since state-of-the-art algorithms for nonconvex MINLP fundamentally rely on solving linear programming (LP) relaxations, we address numerical accuracy directly for LP by means of LP iterative refinement: a new algorithm to solve linear programs to arbitrarily high levels of precision. The thesis is supplemented by an exact extension of the LP solver SoPlex, which proves on average 1.85 to 3 times faster than current state-of-the-art software for solving general linear programs exactly over the rational numbers. These methods can be generalized to quadratic programming. We study their application to numerically difficult multiscale LP models for metabolic networks in systems biology. To improve the computational performance of LP-based MINLP solvers, we show how the expensive, but effective, bound-tightening technique called optimization-based bound tightening can be approximated more efficiently via feasibility-based bound tightening. The resulting implementation increases the number of instances that can be solved and reduces the average running time of the MINLP solver SCIP by 17-19% on hard mixed-integer nonlinear programs. Last, we present branching rules that exploit the presence of nonlinear integer variables, i.e., variables both contained in nonlinear terms and required to be integral. The new branching rules prefer integer variables when performing spatial branching, and favor variables in nonlinear terms when resolving integer infeasibility. They reduce the average running time of SCIP by 17% on affected instances. Most importantly, all of the new methods enable us to solve problems which could not be solved before, either due to their numerical complexity or because of limited computing resources.Gemischt-ganzzahlige nichtlineare Programmierung (MINLP) umfasst die große Klasse endlichdimensionaler mathematischer Optimierungsprobleme aus der gemischt-ganzzahlig linearen Programmierung (MIP) und der globalen Optimierung nichtlinearer Programme (GO). Die Kombination beider Felder erlaubt eine genauere Modellierung vieler Anwendungsprobleme als mit nur einer der beiden Problemklassen möglich. Zugleich ergeben sich daraus aber neue algorithmische Herausforderungen bei der Lösung dieser Modelle. Die vorliegende Arbeit entwickelt neue Methoden zur Verbesserung der numerischen Stabilität und der Leistungsfähigkeit globaler MINLP-Löser. Da leistungsfähige Algorithmen in vielen modernen MINLP-Lösern auf linearer Programmierung (LP) basieren, behandeln wir numerische Genauigkeit direkt auf LP-Ebene. Dazu stellen wir ein neues iteratives Verfeinerungsverfahren für LP vor, das lineare Programme bis auf beliebige Genauigkeit lösen kann. Als Ergänzung zu dieser Arbeit wurde eine exakte Erweiterung des LP-Codes SoPlex entwickelt, die allgemeine lineare Programme mit rationalen Eingabedaten im Durchschnitt 1,85- bis 3-mal schneller löst als mit dem bisherigen Stand der Technik möglich. Wir zeigen, dass diese Algorithmen numerisch schwierige Multiskalenprobleme über metabolischen Netzwerken aus dem Gebiet der Systembiologie verlässlicher lösen als bisher unter Laufzeitbeschränkung möglich. Weiterhin verallgemeinern wir das iterative Verfeinerungsverfahren auf quadratische Programme. Zur Reduktion der Laufzeit LP-basierter MINLP-Löser zeigen wir, wie sich der Effekt des aufwändigen, aber effektiven Verfahrens Optimization-Based Bound Tightening durch das effizientere Feasibility-Based Bound Tightening annähern lässt. Daraus ergibt sich eine Implementierung, die die Anzahl gelöster Instanzen erhöht und die durchschnittliche Laufzeit des MINLP-Lösers SCIP um 17-19% auf schweren MINLPs reduziert. Darüber hinaus entwickeln wir Branching-Regeln, die das Vorhandensein nichtlinearer ganzzahliger Variablen ausnutzen, d.h. von Variablen, die sowohl ganzzahlige Werte annehmen müssen als auch in nichtlinearen Termen von Zielfunktion oder Nebenbedingungen enthalten sind. Dies führt zu einer durchschnittlichen Reduktion der Laufzeit des MINLP-Lösers SCIP um 17% auf relevanten Instanzen. Der wesentlichste Aspekt dieser Verbesserungen besteht jedoch darin, dass durch alle der entwickelten Methoden Probleminstanzen gelöst werden können, deren Lösung zuvor nicht möglich war, sei es aufgrund ihrer numerischen Komplexität oder wegen Beschränkungen an die verfügbare Rechenkapazität
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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