1,720,974 research outputs found
A new extended formulation with valid inequalities for the Capacitated Concentrator Location Problem
Full text linkWe present a new disaggregated formulation of the Capacitated Concentrator Location Problem (CCLP) using the notion of cardinality of terminals assigned to a concentrator. This formulation consists of O(mnn) variables and constraints, where m denotes the number of concentrators and n the number of terminals, respectively. We prove that this extended formulation is stronger than the traditional one. We also present two classes of inequalities exploiting the cardinality effect of the extended formulation. The first class is a generalization of the well-known Cover and (1, k)-Configuration inequalities, which collectively are stronger than the original Cover and (1, k)-Configuration inequalities. The second class, called the 2-Facility Cardinality Matching Inequality, holds for the uncapacitated version of the Concentrator Location Problem and can be lifted to become a strong inequality for CCLP. We solve the LP relaxation of the extended formulation and use separation heuristics to identify and sequentially add the previous valid inequalities to improve the lower bound. This approach is embedded in a branch-and-bound and results in a branch-and-cut approach. We test our solution approach on a large set of benchmark problems. The experimentation shows that we can identify the optimal solution at the root node in most of the problem instances with up to 50 concentrators and 50 terminals. For larger sized test problems with up to 100 concentrators and 1000 terminals, the branch-and-cut procedure using the disaggregated formulation outperforms the branch-and-cut procedure applied to the traditional formulation both in terms of CPU and the required number of branch-and-bound nodes
Separable Lagrangian decomposition for quasi-separable problems
Full text linkLagrangian relaxation is a powerful technique that applies when the removal of some appropriately chosen set of “complicating” constraints makes a(n hard) optimization problem “much easier” to solve. The most common reason for this is that the relaxed problem fully decomposes in (a large number of) independent subproblems. However, a different case happens when the removal of the constraints leaves a number of blocks of semi-continuous variables without constraints between them except those involving the single binary variable commanding them. In this case the relaxation can still be easily solvable, but this involves a two-stage approach whereby the separable blocks are solved first, possibly in parallel, and only then one single problem can be solved to find the optimal value of the design variables. We call this a quasi-separable setting. While the relaxation can be efficiently solved, the fact that it boils down to what formally amounts to a single problem prevents from using techniques—disaggregated master problems, possibly with “easy components”—that allow to solve the corresponding Lagrangian dual more efficiently. We develop an ad-hoc reformulation of the standard master problem of (stabilised) cutting-plane approaches that allow to define the Lagrangian function as the explicit sum of different components, thereby better exploiting the actual structure of the problem, at the cost of introducing a smaller number of extra Lagrangian multipliers w.r.t. what would be required by standard approaches. We also highlight the connection between this reformulation of the master problem and the Lagrangian Decomposition technique. We computationally test our approach on one relevant problem with the required structure, i.e., hard Multicommodity Network Design with budget constraints on the design variables, showing that the approach can outperform state-of-the-art traditional ones
Feature selection in SVM via polyhedral k-norm
Full text linkWe treat the feature selection problem in the support vector machine (SVM) framework by adopting an optimization model based on use of the l pseudo-norm. The objective is to control the number of non-zero components of the normal vector to the separating hyperplane, while maintaining satisfactory classification accuracy. In our model the polyhedral norm ‖. ‖ [k], intermediate between ‖. ‖ 1 and ‖. ‖ ∞, plays a significant role, allowing us to come out with a DC (difference of convex) optimization problem that is tackled by means of DCA algorithm. The results of several numerical experiments on benchmark classification datasets are reported
Ellipsoidal classification via semidefinite programming
Full text linkWe propose a classification approach exploiting relationships between ellipsoidal separation and Support-vector Machine (SVM) with quadratic kernel. By adding a (Semidefinite Programming) SDP constraint to SVM model we ensure that the chosen hyperplane in feature space represents a non-degenerate ellipsoid in input space. This allows us to exploit SDP techniques within Support-vector Regression (SVR) approaches, yielding better results in case ellipsoid-shaped separators are appropriate for classification tasks. We compare our approach with spherical separation and SVM on some classification problems
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
Lo ‘stile immaginifico' di Ernst Jünger nei diari della seconda guerra mondiale (Strahlungen) tra ‘scrittura geroglifica' e ‘nuova teologia'
No full textSaggio sullo “stile immaginifico” di Ernst Jünger nei diari della seconda guerra mondiale (Strahlungen)
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
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