1,720,962 research outputs found
Lattice Quantum Chromodynamics on Intel Xeon Phi based supercomputers
Full text linkPreface
The aim of this master’s thesis project was to expand the QPhiX library for twisted-mass fermions with and without clover term. To this end, I continued work initiated by Mario Schröck et al. [63]. In writing this thesis, I was following two main goals. Firstly, I wanted to stress the intricate interplay of the four pillars of High Performance Computing: Algorithms, Hardware, Software and Performance Evaluation. Surely, algorithmic development is utterly important in Scientific Computing, in particular in LQCD, where it even outweighed the improvements made in Hardware architecture in the last decade—cf. the section about computational costs of LQCD. It is strongly influenced by the available hardware—think of the advent of parallel algorithms—but in turn also influenced the design of hardware itself. The IBM BlueGene series is only one of many examples in LQCD. Furthermore, there will be no benefit from the best algorithms, when one cannot implement the ideas into correct, performant, user-friendly, read- and maintainable (sometimes over several decades) software code. But again, truly outstanding HPC software cannot be written without a profound knowledge of its target hardware. Lastly, an HPC software architect and computational scientist has to be able to evaluate and benchmark the performance of a software program, in the often very heterogeneous environment of supercomputers with multiple software and hardware
layers. My second goal in writing this thesis was to produce a self-contained introduction into the computational aspects of LQCD and in particular, to the features of QPhiX, so the reader would be able to compile, read and understand the code of one truly amazing pearl of HPC [40]. It is a pleasure to thank S. Cozzini, R. Frezzotti, E. Gregory, B. Joó, B. Kostrzewa, S. Krieg,
T. Luu, G. Martinelli, R. Percacci, S. Simula, M. Ueding, C. Urbach, M. Werner, the Intel company for providing me with a copy of [55], and the Jülich Supercomputing Center for granting me access to their KNL test cluster DEE
Matter fields in asymptotically safe quantum field theories of gravity
Full text linkIn this doctoral thesis we present the research of Refs. [1–4] concerning the asymptotic safety
scenario in quantum gravity. We motivate the theoretical and conceptual need for the existence
of a theory of quantum gravity, and explain how such a theory might be realised through the
construction of an ultraviolet complete quantum field theory defined via a high-energy interacting
fixed point. In particular, we give a pedagogical introduction to the functional renormalisation group, as
well as fixed points and critical phenomena. Thereafter we investigate the issue of background
independence, which is introduced when constructing a scale-dependent effective action of a gauge
theory using the background field formalism through the introduction of an infrared cutoff operator
and the gauge fixing procedure. To this end we study simultaneous solutions of the flow equation
combined with so-called modified split Ward identities in a conformally truncated theory in the
derivative expansion. In the main part of this thesis we study the dynamics of a gravitational system coupled to a number of scalar fields. In doing so, we find scaling solutions in a fully functional truncation
using the derivative expansion and background field approximation on a d-dimensional sphere.
We then study a similar system using the vertex expansion in a fully dynamical fluctuation field
calculation. This is done in the exponential as well as the linear parametrisation of the metric,
using different gauge fixing procedures. We find that the overall behaviour is very similar in both
cases. We furthermore find however that the contribution of the scalar fields to the gravitational
coupling differs in its sign compared to older results obtained in a background field approximation.
To correct this behaviour, we supplement the background field equations with modified split Ward
identities, with the aim to obtain the fluctuation field behaviour from a background approximation.
This seems to give satisfactory results in the case of the cosmological constant, whilst the running
of Newton’s constant still differs significantly
Background independence in a background dependent renormalization group
No full textWithin the derivative expansion of conformally reduced gravity, the modified split Ward identities are shown to be compatible with the flow equations if and only if either the anomalous dimension vanishes or the cutoff profile is chosen to have a power-law form. No solutions exist if the Ward identities are incompatible. In the compatible case, a clear reason is found for why Ward identities can still forbid the existence of fixed points; however, for any cutoff profile, a background independent (and parametrization independent) flow equation is uncovered. Finally, expanding in vertices, the combined equations are shown generically to become either overconstrained or highly redundant beyond the six-point level
Asymptotic safety in O(N) scalar models coupled to gravity
Full text linkAbstractWe extend recent results on scalar–tensor theories to the case of an O(N)-invariant multiplet. Some exact fixed point solutions of the RG flow equations are discussed. We find that also in the functional context, on employing a standard “type-I” cutoff, too many scalars destroy the gravitational fixed point. For d=3 we show the existence of the gravitationally dressed Wilson–Fisher fixed point also for N>1. We discuss also the results of the analysis for a different, scalar-free, coarse-graining scheme
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
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