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Orthogonal Eisenstein series at harmonic points and modular forms of singular weight
Get PDFWe investigate the behaviour of orthogonal non-holomorphic Eisenstein series at their harmonic points by using theta lifts. In the case of singular weight, we show that the orthogonal non-holomorphic Eisenstein series that can be written as a theta lift have a simple pole at s=1 whose residues yield holomorphic orthogonal modular forms that are Eisenstein series on the boundary. Moreover, we will investigate the image of this construction and give sufficient conditions for the surjectivity
Tough times for seasoned equity offerings: performance during the COVID pandemic
Get PDFThis study analyzes the wealth effects of SEO announcements in the US during the COVID-19 pandemic and its main determinants. We find significantly negative abnormal returns of − 8.6%. This provides persuasive evidence that capital markets reacted particularly negative during this period, reflecting higher degrees of uncertainty. We furthermore find that larger firms experience a better SEO performance and that COVID-19 related biotech & healthcare firms react particularly negative. This effect is more negative the lower the company valuation beforehand
Fast and reliable transient simulation and continuous optimization of large-scale gas networks
Get PDFWe are concerned with the simulation and optimization of large-scale gas pipeline systems in an error-controlled environment. The gas flow dynamics is locally approximated by sufficiently accurate physical models taken from a hierarchy of decreasing complexity and varying over time. Feasible work regions of compressor stations consisting of several turbo compressors are included by semiconvex approximations of aggregated characteristic fields. A discrete adjoint approach within a first-discretize-then-optimize strategy is proposed and a sequential quadratic programming with an active set strategy is applied to solve the nonlinear constrained optimization problems resulting from a validation of nominations. The method proposed here accelerates the computation of near-term forecasts of sudden changes in the gas management and allows for an economic control of intra-day gas flow schedules in large networks. Case studies for real gas pipeline systems show the remarkable performance of the new method
The sum of its parts: Analysis of federated byzantine agreement systems
Get PDFFederated Byzantine Agreement Systems (FBASs) are a fascinating new paradigm in the context of consensus protocols. Originally proposed for powering the Stellar payment network, FBASs can instantiate Byzantine quorum systems without requiring out-of-band agreement on a common set of validators; every node is free to decide for itself with whom it requires agreement. Sybil-resistant and yet energy-efficient consensus protocols can therefore be built upon FBASs, and the "decentrality" possible with the FBAS paradigm might be sufficient to reduce the use of environmentally unsustainable proof-of-work protocols. In this paper, we first demonstrate how the robustness of individual FBASs can be determined, by precisely determining their safety and liveness buffers and therefore enabling a comparison with threshold-based quorum systems. Using simulations and example node configuration strategies, we then empirically investigate the hypothesis that while FBASs can be bootstrapped in a bottom-up fashion from individual preferences, strategic considerations should additionally be applied by node operators in order to arrive at FBASs that are robust and amenable to monitoring. Finally, we investigate the reported "open-membership" property of FBASs. We observe that an often small group of nodes is exclusively relevant for determining liveness buffers and prove that membership in this top tier is conditional on the approval by current top tier nodes if maintaining safety is a core requirement
The impact of hydrothermal alteration on the physiochemical characteristics of reservoir rocks: the case of the Los Humeros geothermal field (Mexico)
Get PDFHydrothermal alteration is a common process in active geothermal systems and can significantly change the physiochemical properties of rocks. To improve reservoir assessment and modeling of high-temperature geothermal resources linked to active volcanic settings, a detailed understanding of the reservoir is needed. The Los Humeros Volcanic Complex, hosting the third largest exploited geothermal field in Mexico, represents a natural laboratory to investigate the impact of hydrothermal processes on the rock properties through andesitic reservoir cores and outcropping analogs. Complementary petrographic and chemical analyses were used to characterize the intensities and facies of hydrothermal alteration. The alteration varies from argillic and propylitic facies characterized by no significant changes of the REE budget indicating an inert behavior to silicic facies and skarn instead showing highly variable REE contents. Unaltered outcrop samples predominantly feature low matrix permeabilities ( 1.67 W m⁻¹ K⁻¹; > 0.91 10⁻⁶ m² s⁻¹), but a significant loss of magnetic susceptibility (10⁻³–10⁻⁶ SI). In particular, this latter characteristic appears to be a suitable indicator during geophysical survey for the identification of hydrothermalized domains and possible pathways for fluids. The lack of clear trends between alteration facies, alteration intensity, and chemical indices in the studied samples is interpreted as the response to multiple and/or repeated hydrothermal events. Finally, the proposed integrated field-based approach shows the capability to unravel the complexity of geothermal reservoir rocks in active volcanic settings
KORPUSDATEN UND PHRASEODIDAKTIK: PROBLEME UND PERSPEKTIVEN
Get PDFWortverbindungen sind der Schlüssel zu einer kulturell angemessenen Kommunikation. Mithilfe von Korpusdaten kann phraseologisches Wissen nahe am tatsächlichen Sprachgebrauch vermittelt werden. Der vorliegende Artikel stellt die Beiträge der Themenausgabe „Korpora, Phraseologie und DaF“ vor, die den Einsatz von Korpora, ihre hohe Wirksamkeit, aber auch die damit verbundenen Probleme für die Lehr- und Lernpraxis aus unterschiedlichen phraseodidaktischen Perspektiven beleuchten. Folgende Schwerpunkte der Beiträge werden in der Einleitung zusammengefasst: Frequenz als Mittel zur Erfassung besonders typischer und daher bevorzugt zu vermittelnder Phraseme; die Authentizität und Variabilität von Sprachdaten für das Verständnis relevanter Gebrauchsmerkmale; der Nutzen korpusbasierter Analysen für die Entwicklung interkultureller Kompetenz sowie die Möglichkeiten, korpusbasierte Beispiele und Übungen in den Unterricht zu integrieren
TEACHING GERMAN PHRASEMES THROUGH ENGLISH. The role of equivalence
Get PDFThis article delves into the linguistic and pedagogical basis for the PhraseoLab project, whose aim is to promote the learning of German phraseology (collocations, idioms and expressive routine formulae) through English. The methodology adopted in this study, the plurilingual approach, requires the consideration of various phraseological equivalence parameters for the selection of phraseological material for language teaching. Along the qualitative model, aspects related to the prototypical usage of phrasemes must also be taken into account. A corpus-based analysis of frequency applied to collocations is used to refine the traditional model and to provide valuable information in the identification of equivalence levels among phrasemes and in the grading of teaching materials
KORPUS EINFACHES DEUTSCH (KED)
Get PDFDas Korpus einfaches Deutsch (KED) enthält bildungssprachliche Texte in einfacher Sprache, die sich an Lesende mit eingeschränkter Lesekompetenz richten. Das KED ist als Ressource für die datengeleitete Sprachvermittlung im Bereich Deutsch als Fremd- und Zweitsprache (DaF/DaZ) sowie für die korpuslinguistische Forschung zur sprachlichen Komplexität und Einfachheit konzipiert. Im vorliegenden Beitrag wird zunächst die Zusammensetzung des Korpus beschrieben. Im Anschluss wird in einer exemplarischen Anwendung des Korpus im DaF-Unterricht aufgezeigt, wie Belege für die je-desto-Konstruktion gewonnen und didaktisch aufbereitet werden können. Die Daten sind über die KorAP-Anwendung des Leibniz-Instituts für Deutsche Sprache zugänglich. Ein eigenes Suchportal mit sprachdidaktisch orientierten Such- und Filtermöglichkeiten ist in Planung
Detection of spatio-temporal patterns in medical image stacks
Get PDFIn this work, the possibilities of exploiting the medical imaging modality of near-infrared fluorescence optical imaging (NIR-FOI) for the detection of psoriatic arthritis (PsA) are investigated. PsA is an autoinflammatory, chronic disease of the musculoskeletal system, which affects millions of people worldwide. This disease has a wide variety of manifestations such as arthritis with pain, joint stiffness, enthesitis, and more. Therefore, it can reduce the quality of life drastically if left untreated. Even though treatment exists to slow down disease progression, many questions are still unanswered ranging from exact causes over exact definitions to specific disease mechanisms. Thus, researchers across fields are still in the process of defining clear criteria for the definition of this disease as well as developing new detection methods. The unique structure of the measured data requires approaches tailored specifically, not only to the use-case but also to the properties of the data itself. From each measurement, a spatio-temporal image stack with only partially visible objects is received. The general idea is to classify spatio-temporal patterns of the distribution process of an injected color agent. To classify the spatio-temporal patterns a neural network for the classification of individual joints is developed. Nonetheless, additional pre-processing steps are required to ensure the correctness of the measured data. This contains segmentation and artifact detection, which includes three different detection steps: The checking of basic requirements, motion detection, and ambient light detection. Similar to the classification model, the existing approaches do not solve the given task. Besides the approach of classifying the spatio-temporal data a research study was performed to investigate non-pathological factors impacting the acquired images. Finally, two additional applications of NIR-FOI are discussed in the surrounding of developing an entire therapy concept around this imaging modality. One application focuses on therapy monitoring by investigating longitudinal data and in the second application NIR-FOI is merged with a volumetric imaging modality (MRI). For the segmentation and artifact detection, satisfying results are achieved, while the classification still leaves room for improvement. Furthermore, the investigation of non-pathological factors reveals some parameters with an impact on the imaging data but the main driver for the data heterogeneity of different patients cannot be identified. The therapy monitoring approach does not allow a final conclusion if NIR-FOI is suitable for that task but it can be shown that even in the longitudinal data a normalization step needs to be included since the data across multiple visits is not directly comparable. Furthermore, the necessary steps for such a monitoring system are introduced. Finally, a well-working registration for combining NIR-FOI and MRI is presented. Although the different steps in this thesis achieve very different results, the groundwork for developing a comprehensive well-working analysis approach for the detection of PsA in NIR-FOI data is presented in this thesis
Local Temperatures at Electric Contacts in Ball Bearings at Mixed Lubrication
Get PDFThis thesis focuses on the tribological contacts in a laboratory setup with one axial ball bearing, at mixed lubrication, exposed to DC currents above 0.5 A. This work gives the bearing electrical characteristics and the influencing factors in the bearing fluting generation.
The tribological contact between the ball and the raceway of the axial ball bearing type 51208 is described with a focus on several relevant parameters (such as bearing speed, bearing force, and bearing current) for the bearing electric behavior. The surface roughness and the covering oxide layer on the surface of the ball and of the raceway in the Hertz’ian area mainly determine the electrical behavior. White light interferometry (WLI) and X-ray photoelectron spectroscopy (XPS) are used to study the roughness and oxide layer, respectively.
The established knowledge of electric contacts, developed for mechanical switches, is used to describe the measured bearing voltage u_b and bearing current i_b. It turns out that the electric current flows only through tiny bottle-necked areas within the Hertz’ian area, resulting in an electrical constriction resistance and the generation of so-called a-spots (according to R. Holm). The A-fritting and B-fritting of the oxide layer are explained in connection with the measured voltage and current signals. It is revealed that with a non-interrupted bearing current (DC or AC) with an amplitude of typically more than 0.5 A, the bearing voltage remains at values of around 1 V. This behavior resembles antiparallel diodes with a forward voltage of about the fritting voltage.
The temperature-rise ∆T_c, due to the electric current flow at a local metal-metal contact (a-spot), in the Hertz’ian area is analytically and numerically calculated. The finite element simulation results agree with the analytical calculations. Assuming identical electric contacts for all the ball-raceway contacts of the axial ball bearing at mixed lubrication, a bearing voltage of 0.8 V results in local contact temperatures of ca. 1035 °C in less than 1 µs. The findings show that the melting temperature (ca. 1400 °C) and softening temperature (ca. 800 °C) of the bearing steel occur at respectively a bearing voltage of 1.05 V and 0.65 V. At a continuous bearing current, with an amplitude above 0.5 A, bearing voltages in the range 0.65–1.05 V are typically measured, implying the existence of local electric contact points with these locally elevated temperatures. The blackened (burned) lubricant, and the imprints of local melting points in scanning electron microscopy (SEM) of the surface agree with these calculated elevated temperatures.
The electrical characteristic of the ball-raceway contact is similar to the Ring-Ring metal contact in a second laboratory setup, investigated in this work. With the steel Ring-Ring setup, which is a single rolling contact, the fluting is generated as well as in the bearings. The measured contact voltage in the Ring-Ring setup is almost equal to the ball-raceway contact voltage in the bearing. With rings of carbon steel, aluminum, and copper a contact voltage is observed, similar to the bearing steel. The steel Ring-Ring contact shows that with an increased thickness of the oxide layer, e.g. in a high temperature furnace, the contact voltage increases, and with a decreased thickness of the oxide layer, e.g. using acid, the contact voltage decreases. Unlike steel rings, the contact voltage between two silver rings is almost zero, due to the absence of an oxide layer on the silver rings.
A study of parameters (such as bearing speed, bearing force, bearing temperature, and bearing current amplitude) on the fluting generation with HF bearing currents at voltage-source inverter operation is generally time-consuming, because the duration of electric current conduction at each inverter switching incident is generally much shorter than of a continuous DC and AC current. In this work, the fluting generation is investigated mainly with DC currents mostly at high current densities above 1 A/mm2. Changing the electrical and mechanical parameters resulted in advancing or postponing the fluting generation. Despite an apparent current density above 1 A/mm2, in some cases no fluting is generated within the selected test duration. An increased bearing current density or increased bearing speed, and a decreased bearing force or decreased bearing temperature, increase the chance of fluting. With DC currents, the fluting almost always appears first on the ring with the negative polarity.
With AC currents (up to 1 MHz) no fluting is observed after up to 168 hours of operation. With DC currents, having the equal amplitude to the AC currents, the fluting is generated in less than 24 hours. In tests with a DC current, the oxide layer thickness on the negative ring may be reduced due to the direction of the electric field. Measurements in long-term tests of up to 168 hours with DC currents, show that the negative ring (except at a very high current density of 58.6 A/mm2) loses more mass than the positive ring. Unlike the negative ring, the positive ring in some cases even gains additional mass. Rectifying the discharge bearing currents resulted in bearing fluting at an early time on the negative rings of the bearings in an inverted-fed induction machine of 90 W.
A test with a deliberately created pit on the raceway was run to cause an additional lateral vibration of the ball on the raceway, in the absence of any bearing current. This test did not result in any fluting. The fluting on a fluted bearing does not relocate or develop in the absence of bearing currents. Hence, the electric current flow is essential in addition to the lateral vibrations for the fluting generation. Tests without lubrication in the Ring-Ring setup result in no fluting. Hence, an elastic medium, like the lubrication film, is essential for the lateral vibrations of the rings. The hardness of the material is essential for the fluting generation, as with copper and silver no fluting is observed. With aluminum, the fluting is observed first on the positive ring. An increased slide-to-rolling ratio (ratio of sliding speed to rolling speed) in the Ring-Ring contact changed the shape of flutings pattern.
This work achieved a reasonable explanation for the high local temperatures in the Hertz’ian area at an electric current flow in mixed lubrication. A variety of tests are conducted and interpreted to understand the electrical characteristic of the bearing, based on the well-established knowledge of classical electric contacts in mechanical switches. Various test conditions are investigated to enlighten the fluting generation mechanism. The results are explained to the best of the author’s knowledge. Nevertheless, no proof or completeness of the fluting generation mechanism is claimed. Here, still further detailed investigation on the current flow at moving and vibrating electrical contact together with a simultaneous change of the surface properties of the contact partners are necessary