1,720,995 research outputs found
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
Dynamic Graph Generation and an Asynchronous Parallel Bundle Method Motivated by Train Timetabling
Full text linkLagrangian relaxation is a successful solution approach for many combinatorial optimisation problems, one of them being the train timetabling problem (TTP). We model this problem using time expanded networks for the single train schedules and coupling constraints to enforce restrictions like station capacities and headway times. Lagrangian relaxation of these coupling constraints leads to shortest path subproblems in the time expanded networks and is solved using a proximal bundle method. However, large instances of our practical partner Deutsche Bahn lead to computationally intractable models. In this thesis we develop two new algorithmic techniques to improve the solution process for this kind of optimisation problems.
The first new technique, Dynamic Graph Generation (DGG), aims at improving the computation of the shortest path subproblems in large time expanded networks. Without sacrificing any accuracy, DGG allows to store only small parts of the networks and to dynamically extend them whenever the stored part proves to be too small. This is possible by exploiting the properties of the objective function in many scheduling applications to prefer early paths or due times, respectively. We prove that DGG can be implemented very efficiently and its running time and the size of nodes that have to be stored additionally does not depend on the size of the time expanded network but only on the length of the train routes.
The second technique is an asynchronous and parallel bundle method (APBM). Traditional bundle methods require one solution of each subproblem in each iteration. However, many practical applications, e.g. the TTP, consist of rather loosely coupled subproblems. The APBM chooses only small subspaces corresponding to the Lagrange multipliers of strongly violated coupling constraints and optimises only these variables while keeping all other variables fixed. Several subspaces of disjoint variables may be chosen simultaneously and are optimised in parallel. The solutions of the subspace problem are incorporated into the global data as soon as it is available without any synchronisation mechanism. However, in order to guarantee convergence, the algorithm detects automatically dependencies between different subspaces and respects these dependencies in future subspace selections. We prove the convergence of the APBM under reasonable assumptions for both, the dual and associated primal aggregate data. The APBM is then further extended to problems with unknown dependencies between subproblems and constraints in the Lagrangian relaxation problem. The algorithm automatically detects these dependencies and respects them in future iterations. Again we prove the convergence of this algorithm under reasonable assumptions.
Finally we test our solution approach for the TTP on some real world instances of Deutsche Bahn. Using an iterative rounding heuristic based on the approximate fractional solutions obtained by the Lagrangian relaxation we are able to compute feasible schedules for all trains in a subnetwork of about 10% of the whole German network in about 12 hours. In these timetables 99% of all passenger trains could be scheduled with no significant delay and the travel time of the freight trains could be reduced by about one hour on average
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
A Polyhedral Study of Quadratic Traveling Salesman Problems
Full text linkThe quadratic traveling salesman problem (QTSP) is an extension of the (classical) Traveling Salesman Problem (TSP) where the costs depend on each two nodes that are traversed in succession, i. e., on the edges in the symmetric (STSP) and on the arcs in the asymmetric case (ATSP). The QTSP is motivated by an application in bioinformatics. It can be used in the solution of certain Permuted Markov models that are set up for the recognition of transcription factor binding sites and of splice sites in gene regulation. Important special cases are the Angular-Metric TSP used in robotics and the TSP with Reload Costs used in the planning of telecommunication and transport networks.
The SQTSP and the AQTSP can be formulated as integer optimization problems over the polytope associated with the STSP resp. ATSP together with a quadratic cost function. We study the polytopes arising from a linearization of the respective quadratic integer programming formulations. Based on the proof of the dimension of the polytopes using the so called direct method we can prove the facetness of several valid inequalities. These facets and valid inequalities can be divided into three large groups. Some are related to the Boolean quadric polytope. Furthermore we introduce the conflicting edges/arc inequalities that forbid certain configurations of edges and 2-edges resp. of arcs and 2-arcs. Finally, we strengthen valid inequalities of STSP and ATSP in order to get stronger inequalities in the quadratic case. We present two general lifting approaches. One is applicable to all inequalities with nonnegative coefficients and the second allows to strengthen clique tree inequalities. Applying these approaches to the subtour elimination constraints leads to facets in most cases, but in general facetness is not preserved. In addition, the complexity of the separation problems for some of the facet classes is studied.
Finally, we present some computational results using a branch-and-cut framework, which is improved by some of the newly derived cutting planes. The tested instances from biology could be solved surprisingly well. Instances with up to 100 nodes could be solved in less than 700 seconds improving the results in the literature by several orders of magnitude. For most of the randomly generated instances using some additional separators allowed to reduce the root gaps and the numbers of nodes in the branch-and-cut tree significantly, often even the running times
Lösungsmethoden für gemischt-ganzzahlige lineare und nichtlineare Netzwerkoptimierungsprobleme basierend auf lokalen Reformulierungen und Relaxierungen
Full text linkSince the beginnings of network optimization, the number of use cases has grown enormously and can be expected to further expand in an increasingly interconnected world. The wide range of modern applications include optimization tasks on energy networks, telecommunication networks and in public transport, just to name a few. Although many traditional network optimization problems are NP-hard in their basic version, applications pose additional challenges due to more complicated — often nonlinear — dependencies or the sheer size of the network. In this thesis, we develop methods that help to cope with those challenges. A common strategy will be to improve mathematical programming formulations locally by modeling substructures in an integrated way. The resulting reformulations and relaxations will allow for global methods that either solve the problem to exact optimality or up to a predefined precision. For large-scale network expansion problems, a solution method is proposed that is based on iterative aggregation. Starting with an initial aggregation, we solve a sequence of network design problems over increasingly fine-grained representations of the original network. This is done until the whole network is represented sufficiently well in the sense that an optimal solution to the aggregated problem can easily be extended to an optimal solution of the original problem. Global optimality is guaranteed by a subproblem that computationally is less expensive and either proves optimality or gives an indication of where to refine the representation. In this algorithmic scheme, locally relaxing the problem allows us to focus on the critical part of the network. In many optimization problems on transportation networks — especially those arising from energy applications — the main challenge is connected to the problem’s nonlinear features, arising, for example, from laws of physics. Gas networks represent a typical example for such a nonlinear network flow setting that we will repeatedly refer to throughout this work. A common and established solution approach consists of constructing a piecewise linear approximation or relaxation. We study how to strengthen the resulting mixed-integer programming formulation for specific substructures in the network. We find effective cutting planes and derive a complete description for induced paths of arbitrary length — using graph-theoretic arguments related to perfect graphs. A generalization of key properties of this special case leads to an abstract definition in terms of clique problems on a specific type of graph. This abstract setting also comprises a basic version of the project scheduling problem and still allows us to give totally unimodular reformulations that are of linear size. Moreover, questions regarding recognizability of this structure will be discussed. We also discuss the concept of simultaneous convexification that can be seen as a continuous counterpart to our approach for piecewise linearized problems. The resulting reformulations can improve relaxations employed by general-purpose MINLP solvers, which usually rely on convexifying nonlinear functions separately. Computational results demonstrate the practical impact of the methods developed in this thesis, in many cases using real-world data sets.Seit den Anfängen der Netzwerkoptimierung ist die Zahl der Anwendungsfälle immens gewachsen und angesichts einer zunehmend vernetzten Welt ist ein weiterer Anstieg zu erwarten. Die Spannbreite moderner Anwendungen umfasst Optimierungsprobleme auf Energienetzen, Telekommunikationsnetzen und Verkehrsnetzen, um nur einige zu nennen. Auch wenn viele traditionelle Netzwerkoptimierungsprobleme bereits in ihrer Grundversion NP-schwer sind, stellen Anwendungen weitere Anforderungen aufgrund komplexerer - oftmals nichtlinearer - Abhängigkeiten oder der schieren Größe der zugrunde liegenden Netzwerke. In dieser Arbeit werden Methoden entwickelt, um mit diesen Herausforderungen umzugehen. Die wesentliche Strategie wird darin bestehen, mathematische Problemformulierungen lokal zu verstärken, indem ausgewählte Substrukturen als Ganzes erfasst und modelliert werden. Die resultierenden Reformulierungen und Relaxierungen unterstützen globale Methoden, die entweder exakte oder bis auf eine vordefinierte Genauigkeit optimale Lösungen finden. Für große Netzausbauprobleme wird eine Lösungsmethodik basierend auf iterativer Aggregation entworfen. Beginnend mit einer Startaggregation lösen wir eine Folge zunehmend detaillierter Vergröberungen des ursprünglichen Netzwerks, bis die Darstellung hinreichend genau ist, sodass seine Optimallösung leicht auf das ursprüngliche Problem übertragen werden kann. Exaktheit wird dabei durch ein Subproblem sichergestellt, das entweder Optimalität bestätigt oder Ansatzpunkte zur Verfeinerung der Darstellung liefert. In diesem Schema erlaubt somit eine lokale Relaxierung die Fokussierung auf kritische Teile des Netzwerks. In vielen Optimierungsproblemen auf Transportnetzen, insbesondere für Energieträger, besteht die wesentliche Herausforderung in den auftretenden Nichtlinearitäten, die beispielsweise physikalischen Gesetzen geschuldet sind. Gasnetwerke sind hierfür ein typisches Beispiel, auf das wir uns mehrfach in dieser Arbeit beziehen werden. Ein etablierter Lösungsansatz besteht in der Konstruktion stückweise linearer Approximationen oder Relaxierungen. Es wird untersucht, wie die entstehende Formulierung für bestimmte Substrukturen verstärkt werden kann. Dabei finden wir effektive Schnittebenen und leiten mittels graphentheoretischer Argumente eine vollständige Beschreibung für induzierte Pfade her. Eine Abstraktion wesentlicher Eigenschaften dieses Spezialfalls führt auf Cliquenprobleme auf bestimmten Graphen. Dieser abstrakte Rahmen umfasst auch eine Basisversion des Projektplanungsproblems und erlaubt es weiterhin, eine total unimodulare Reformulierung von linearer Größe nachzuweisen. Weiterhin werden Fragen zur Erkennbarkeit dieser Struktur behandelt. Außerdem wird das Konzept der simultanen Konvexifizierung diskutiert, das als kontinuierliches Gegenstück zu unserem Ansatz für stückweise linearisierte Probleme angesehen werden kann. Die entstehenden Reformulierungen verstärken Relaxierungen, auf die allgemeine MINLP-Löser typischerweise angewiesen sind. Rechenergebnisse unter Einbeziehung realer Datensätze zeigen den praktischen Einfluss der in dieser Arbeit entwickelten Methoden
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