1,355,106 research outputs found
Learning to Schedule Heuristics in Branch and Bound
No full textPrimal heuristics play a crucial role in exact solvers for Mixed Integer Programming (MIP). While solvers are guaranteed to find optimal solutions given sufficient time, real-world applications typically require finding good solutions early on in the search to enable fast decision-making. While much of MIP research focuses on designing effective heuristics, the question of how to manage multiple MIP heuristics in a solver has not received equal attention. Generally, solvers follow hard-coded rules derived from empirical testing on broad sets of instances. Since the performance of heuristics is problem-dependent, using these general rules for a particular problem might not yield the best performance. In this work, we propose the first data-driven framework for scheduling heuristics in an exact MIP solver. By learning from data describing the performance of primal heuristics, we obtain a problem-specific schedule of heuristics that collectively find many solutions at minimal cost. We formalize the learning task and propose an efficient algorithm for computing such a schedule. Compared to the default settings of a state-of-the-art academic MIP solver, we are able to reduce the average primal integral by up to 49% on two classes of challenging instances
Elevated expression of the V-ATPase C subunit triggers JNK-dependent cell invasion and overgrowth in a Drosophila epithelium
No full textSUMMARY
The C subunit of the vacuolar H+-ATPase or V-ATPase regulates the activity and assembly of the proton pump at cellular membranes. It has been shown to be strongly upregulated in oral squamous cell carcinoma, a highly metastatic epithelial cancer. In addition, increased V-ATPase activity appears to correlate with invasiveness of cancer cells, but the underlying mechanism is largely unknown. Using the Drosophila wing imaginal epithelium as an in vivo model system, we demonstrate that overexpression of Vha44, the Drosophila orthologue of the C subunit, causes a tumor-like tissue transformation in cells of the wing epithelium. Overexpressing cells are excluded from the epithelium and acquire invasive properties while displaying high apoptotic rates. Blocking apoptosis in these cells unmasks a strong proliferation stimulus, leading to overgrowth. Furthermore, we show that excess Vha44 greatly increases acidification of endocytic compartments and interferes with endosomal trafficking. As a result, cargoes such as GFP-Lamp1 and Notch accumulate in highly acidified enlarged endolysosomal compartments. Consistent with previous reports on the endocytic activation of Eiger/JNK signaling, we find that V-ATPase stimulation by Vha44 causes JNK signaling activation whereas downmodulation of JNK signaling rescues the invasive phenotypes. In summary, our in vivo-findings demonstrate that increased levels of V-ATPase C subunit induce a Eiger/JNK-dependent cell transformation within an epithelial organ that recapitulates early carcinoma stages
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Soil microbial carbon turnover decreases with increasing molecular size
No full textIt is well established that soil microorganisms play an important role in respiration of newly fixed plant carbon. Recent results show that they also contribute significantly to soil organic matter (SOM) formation. We hypothesized that different molecular size classes of compounds in soil microbial biomass (SMB) have variable turnover time and in consequence influence SOM formation differentially. Here we used natural differences in carbon stable isotope signatures (d13C values) after C3eC4 vegetation change to track newly fixed C4 plant carbon into SMB molecular size classes. SMB was obtained by chloroform fumigation extraction (SFE) and d13C values of its size classes were measured using size exclusion chromatography coupled online to liquid chromatography‒isotope ratio mass spectrometry (SECeLC eIRMS). Resolved SMB was assigned to 5 size classes of 1800e9800, 800e1800, 380e800, 180e380 and 50e180 Da respectively. The contribution of recent C4 plant carbon to size classes of SMB decreased with increasing molecular weight (MW). It ranged from 77 19% in the lowest MW size class size class to 41 14% in the highest MW size class in a sandy soil and from 59 18% in the lowest MW size class to 8 15% in the highest MW size class in a clayey soil. A decreasing carbon turnover of compounds in SMB extracts along a continuum of molecular size from small to large implies that low molecular weight microbial compounds are rapidly metabolized products that link to fast respiratory carbon fluxes, whereas high molecular weight ones could be products of microbial synthesis like structural compounds that have slower turnover rates and link to slower SOM formation. Our methods help avoid contamination of CFE extracts and the results help explain why SMB turnover is faster in CFE extracts when compared to calculations using membrane lipids (e.g. PLFA-based)
Variations on the Author
No full text“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
No full textWe 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
Overstory-specific effects of litter fall on the microbial carbon turnover in a mature deciduous forest
No full textMature deciduous forests can serve as important carbon (C) sinks, but the C storage differs significantly in dependency on the tree species. To specify the significance of overstory-specific effects of litter fall on the soil microbial C turnover, we have investigated the C-13 isotopic signature of microbial biomarker phospholipid fatty acids (PLFAs). Samples were taken under pure Fagus sylvatica and mixed overstory (F. sylvatica and Fraxinus excelsior or F. excelsior, Acer spp. and F. sylvatica) in a mature temperate deciduous forest in Central Germany 4 weeks prior to and 3 weeks after litter fall. Accordingly, the CO2 emission from soil was measured before, during and after the litter fall to investigate the response of decomposition. At all sites and at both sampling dates the fungal biomarker PLFA 18:2 omega 6,9 had predominantly lower delta C-13 values (from -32 to -43 parts per thousand) than the bacterial biomarker PLFAs (delta C-13 values from -23 to -39 parts per thousand). This difference indicated that fungi generally used preferentially plant derived C, whereas the bacterial populations include groups which used SOM derived C, independent on the overstory, trees. Under pure F. sylvatica overstory the delta C-13 values of microbial biomarker PLFAs were slightly decreased (up to 2 parts per thousand for 17:0br) or unchanged after litter fall. By contrast, under both variants of mixed overstory the delta C-13 values of biomarker PLFAs of fungi (18:2w6,9) were increased after litter fall (+3.5 and +3.8 parts per thousand). This might be explained partly by a faster initial decomposition of foliar litter from mixed overstory already during litter fall as confirmed by higher CO2 emission under mixed F. excelsior, Acer spp. and F. sylvatica than under pure F sylvatica in this period. However, the involved microbial populations differed overstory-specific. Bacterial biomarker PLFAs with strongest overstory-specific differences in the response on litter fall were 17:0br (Gram-positive bacteria), 18:1 and 19:0cy (Gram-negative bacteria). The present results indicate that a tree species conversion even exclusively between deciduous tree species might alter the soil microbial C turnover during litter decomposition and suggest that it would in the long-term change the SOM stability and C storage. (C) 2009 Elsevier B.V. All rights reserved. [References: 54
On generalized surrogate duality in mixed-integer nonlinear programming
No full textThe most important ingredient for solving mixed-integer nonlinear programs (MINLPs) to global ε-optimality with spatial branch and bound is a tight, computationally tractable relaxation. Due to both theoretical and practical considerations, relaxations of MINLPs are usually required to be convex. Nonetheless, current optimization solvers can often successfully handle a moderate presence of nonconvexities, which opens the door for the use of potentially tighter nonconvex relaxations. In this work, we exploit this fact and make use of a nonconvex relaxation obtained via aggregation of constraints: a surrogate relaxation. These relaxations were actively studied for linear integer programs in the 70s and 80s, but they have been scarcely considered since. We revisit these relaxations in an MINLP setting and show the computational benefits and challenges they can have. Additionally, we study a generalization of such relaxation that allows for multiple aggregations simultaneously and present the first algorithm that is capable of computing the best set of aggregations. We propose a multitude of computational enhancements for improving its practical performance and evaluate the algorithm’s ability to generate strong dual bounds through extensive computational experiments
On Generalized Surrogate Duality in Mixed-Integer Nonlinear Programming
No full textDue to both theoretical and practical considerations, relaxations of MINLPs are usually required to be convex. Nonetheless, current optimization solvers can often successfully handle a moderate presence of nonconvexities, which opens the door for the use of potentially tighter nonconvex relaxations. In this work, we exploit this fact and make use of a nonconvex relaxation obtained via aggregation of constraints: a surrogate relaxation. These relaxations were actively studied for linear integer programs in the 70s and 80s, but they have been scarcely considered since. We revisit these relaxations in an MINLP setting and show the computational benefits and challenges they can have. Additionally, we study a generalization of such relaxation that allows for multiple aggregations simultaneously and present the first algorithm that is capable of computing the best set of aggregations. We propose a multitude of computational enhancements for improving its practical performance and evaluate the algorithm’s ability to generate strong dual bounds through extensive computational experiments
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