1,721,074 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Synchronization phenomena in complex networks of pulse-coupled oscillators: self-generated changes between randomness and order
No full textComplex network comprised of interconnected oscillatory systems are investigated in various contexts in the natural sciences. It is an important goal to understand in which way synchronization phenomena in such networks are determined by the structure of the network itself and by properties of the oscillatory systems. Contrasting the microscopic behavior which often shows fluctuations, many studies have characterized synchronization and desynchronization as convergence towards oscillatory and constant macroscopic behavior, respectively. However, very few models have been developed which show self-generated and recurrent changes between randomness and order. In this thesis, we study the influence of the connection structure on the dynamical behavior of complex networks of oscillators. More concretely, we consider delta-pulse-coupled, excitatory integrate-and-fire-like oscillators, possibly endowed with refractory periods and time delayed interactions, and study their dynamics when coupled onto random and onto small-world networks, i.e., networks with both short-ranged and long-ranged connections. In random networks, these oscillators may exhibit both chaotic, asynchronous behavior and synchronous behavior. We define and investigate different types of continuum models which describe the thermodynamic limit of large networks and allow us to characterize different mechanism for the loss of stability of asynchronous behavior. If stability is lost, oscillators may show partially synchronous states and irregular behaviors. We relate the latter to recurrent times at which the firing rate in the networks diverges and a macroscopic amount of oscillators spontaneously fires in unison. Continuing one of these transitions to synchrony into the small-world regime leads to a rich spectrum of dynamical behaviors which are determined by the interplay between pattern formation and synchronization phenomena. In addition to various kinds of wave-dominated states, we report here on so-called chimera states, in which the networks split into asynchronous and synchronous regions. Depending on oscillator parameters, these regions may be fixed in shape and size, or may evolve constantly leading to macroscopic observables which show non-converging behavior. The latter may take the form of asynchronous firing interrupted by short and rare events of synchronous firing, or of a network rhythm in which more and less synchronous behaviors alternate on a timescale well below the period duration of oscillators. The non-converging synchronization phenomena that we describe in this thesis add new aspects to the discussion which may help in unraveling the complicated relationships between the network structure, node dynamics, and the collective network dynamics
Lorentzkraftbildgebung : MR-basierte Darstellung Neuronaler Aktivität in Echtzeit
Get PDFIn dieser Arbeit wurde eine neue Methode zur Darstellung neuronaler Aktivität in Echtzeit vorgestellt. Dabei wird die Lorentzkraft, die auf die elektrischen Ladungsträger aktiver Nervenzellen im Magnetfeld eines MR-Tomographen wirkt, durch eine externe Bewegung, die die Geschwindigkeit der Ladungsträger im Laborsystem erhöht, verstärkt. Diese Lorentzkraft führt im weichen Hirngewebe zu einer Verschiebung, die mittels MR-Phasenbildgebung dargestellt wird. Es wurde ein Prototyp zum induzieren dieser Bewegung im MR-System und die Ansteuerung und Synchronisierung der Bewegung beschrieben. Als Proof of Principle wurde die Verarbeitung optischer Reize im Gehirn an zwei gesunden Probanden untersucht und mit unter gleichen Bedingungen durchgeführten EEG-Messungen verglichen. Dabei zeigen die Messungen die erwartete Gewebeverschiebung im Bereich der primären Sehrinde, deren zeitliches Verhalten mit dem der EEG-Messungen übereinstimmt
MR-Phasenbilder ultraschallinduzierter Verschiebungsfelder an Brustläsionen
Get PDFIn dieser Arbeit wurde eine Methode erforscht, die die Uktraschalstrahlungskraft mit der MRT kombiniert. Durch die Schallstrahlungskraft werden im Gewebe Verschiebungen hervorgerufen. Diese lieferten einen Kontrast in MR-Phasenbildern, welche mit einer bewegungssensitiven Spin-Echo-Sequenz aufgenommen wurden. Aus den Phasenbildern konnte pixelweise die Verschiebung in Schallausbreitungsrichtung berechnet werden, welche maßgeblich von der Schallabsorption und den elastischen Eigenschaften des Gewebes abhängt. In vorbereitenden Messungen an einem kommerziellen Phantom und mehreren selbst hergestellten wurde das Verhalten von homogenem Gewebe, weichen und festen Einschlüssen sowie Kalkeinschlüssen unter Einfluss der Schallstrahlungskraft von Ultraschall untersucht. Die Einschlüsse wurden lokalisiert und das charakteristische Verhalten erklärt. Da im Rahmen dieser Arbeit ein Ethik-Antrag bei den zuständigen Ethikkommissionen erfolgreich gestellt wurde, konnten die Erkenntnisse aus den Phantommessungen auf Messungen an Versuchspersonen übertragen werden. Es wurden drei Patientinnen mit verschiedenen Pathologien (eine Zyste und zwei Fibroadenome) untersucht. In allen Messungen kam es im Gewebe zunächst zu einem Anstieg der Verschiebung mit anschließendem Abfall, was auf die Randbedingungen zurückzuführen ist. Beim Übergang ins zystische Gewebe kam es erneut zu einem Anstieg der Verschiebung, da es weicher als das umliegende Gewebe ist. Obwohl das Gewebe des Fibroadenoms fester als das umliegende Gewebe ist, wird es als Ganzes stärker verschoben als die Umgebung. Dieses Verhalten lässt sich mit medizinischem Hintergrundwissen erklären. Bei Fibroadenomen handelt es sich zwar um Verhärtungen, diese sind allerdings abgekapselt und im umliegenden Gewebe leicht verschieblich. Trifft der Ultraschall auf das Fibroadenom, so kommt es zur Reflektion an der Grenzfläche zwischen dem Drüsengewebe und dem Fibroadenom. Der so erzeugte erhöhte Kraftübertrag führt zur gesteigerten Verschiebung des gesamten Fibroadenoms. Parallel zu den Messungen an Patientinnen wurde eine Störungsanalyse an gesunden Versuchspersonen durchgeführt. Diese ergab, dass die auftretenden Bildstörungen maßgeblich durch ungewollte Bewegungen des Messvolumens hervorgerufen werden. Die Ergebnisse zeigen, dass die vorgestellte Methode erfolgreich an Patientinnen angewendet werden kann und das Potenzial hat, ohne jegliches Gesundheitsrisiko das Spektrum der Vorsorgeuntersuchungen zu erweitern. Vorhandene Befunde wurden bestätigt, was darauf hoffen lässt, dass diese Methode, nach einer Weiterentwicklung und in Kombination mit den Standard-Techniken, zur Verbesserung der Brustkrebsdiagnostik beitragen kann und damit falsch-positive Befunde und Biopsieuntersuchungen reduziert werden können
Hadron-Hadron Interactions from <em>N<sub>f</sub> = 2+1+1</em> Lattice QCD : The <em>ρ</em>-resonance
No full textIn this work the calculation of the ρ resonance's decay parameters with Nf = 2+1+1 flavour lattice QCD is presented. The calculation is performed based on gauge configuration ensembles produced by the ETM collaboration which were generated with three different lattice spacing values and pion masses ranging from 230 MeV to 500 MeV. The calculation of resonance parameters with Lattice QCD requires correlation functions of all relevant decay channels in multiple moving reference frames. In this work operators resembling a ρ meson as well as a p+p system are used. The boost to moving frames breaks rotational symmetry and thereby causes a level splitting. Operators which transform like basis states of the reduced symmetry groups' irreducible representations are constructed to determine each energy level individually. Aided by the stochastic Laplacian Heaviside method correlation functions are calculated for all lattice momenta up to (0, 0, 2) and all irreducible representations that emerge. From these correlation functions energy levels are determined under consideration of systematic error sources. Most notably the effect of thermal pollutions and bias from fit range selection are taken into account. By applying the Lüscher method the energy spectra are translated into phase shift curves on each ensemble separately. From a Breit-Wigner fit to the phase shift curves the ρ meson mass and width on all ensembles are determined. The results are fed into a combined fit of mass and width and extrapolated to the chiral and continuum limit. The main result of this thesis are the continuum extrapolated values of Mρ and Γρ at the physical point which were determined to Mρ = 769(19) MeV and Γρ = 129(7) MeV. Lattice artefacts could not be resolved within the statistical uncertainties of this work. While the ρ meson mass is in very good agreement with experiment the corresponding decay width differs by about two standard deviations from its experimental counterpart
Transverse Momentum Dependent Parton Distribution Functions from Twisted Mass Lattice QCD
Get PDFThe aim of this thesis is the theoretical calculation of the unpolarized transverse momentum dependent parton distribution function (TMDPDF) from first principles using lattice QCD. TMDPDFs are important quantities that provide information about the three-dimensional structure of the nucleon. Experimentally, they can be accessed through the Drell-Yan and the semi-inclusive deep inelastic scattering processes. Presently, this way of extracting TMDPDFs, through global fits to experimental data, lacks in precision. This is expected to improve with more data from future experiments and in particular from the planned Electron-Ion Collider (EIC). A theoretical computation of TMDPDFs can serve as an input to future phenomenological studies and also as a guideline for observations at EIC.
The lattice QCD calculation of TMDPDFs is challenging due to partonic observables being defined on the light-cone. The development of the large momentum effective theory (LaMET) has been one of the most successful approaches to tackle this problem. In this thesis, we present an exploratory study of the calculation of the unpolarized TMDPDF in the LaMET framework using the twisted mass fermion action. We use two lattice ensembles at same lattice spacing, {F_DISS_ABSTRACT}.09 and ^3 times 64 MeV and 0$ MeV respectively. We calculate the three quantities necessary for the extraction of the TMDPDF, namely the quasi-TMDPDF, the Collins-Soper kernel and the reduced soft function. We also study the renormalization of the underlying staple-shaped Wilson line operator using regularization independent momentum subtraction (RI/MOM) scheme. We find a vanishingly small contribution from mixing and study alternate multiplicative renormalization methods such as the short distance ratio and the RI-short. We finally perform a consistent matching up to 1-loop in perturbation theory to obtain the physical TMDPDF. We find our results to be comparable to previous calculations on the lattice and also to the phenomenological fits
Quark-mass dependence of pion scattering amplitudes
Get PDFWe analyze lattice-QCD data of the ππ → ππ P wave obtained at unphysical-high pion masses with the inverse-amplitude method at next-to-leading order and at next-to-next-to-leading order. We then extrapolate to the physical pion mass and determine the properties of the ρ resonance from its pole in the complex plane. By comparing both orders, we estimate the systematic error associated with the truncation. Subsequently, we use the inverse-amplitude method as input to Khuri-Treiman equations for γ(*)π → ππ. The resulting dispersive framework is fit to lattice-QCD data at unphysical pion masses and then again extrapolated to the physical mass, where we determine the radiative coupling of the ρ, the lineshape, and the chiral anomaly of the process. Lastly, we develop a method to assess the statistics needed in experiments to be sensitive to higher-order pion rescattering in decays of a single particle into three pions and illustrate the approach for simple cases
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