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Counting and sliding verifying and restoring healthy systems
No full textReactive systems play a significant role in the daily life of every person. Such systems
include personal computers, automatic teller machines, devices we use every day in our
households, and also systems that play a less active but still important role, for example
monitoring devices scanning for environmental threats in a country. In particular for
safety-critical reactive systems it is of enormous importance that correctness properties of
such systems have been verified. A very prominent technique to tackle such verification
tasks is called model checking. From a theoretical point of view, model checking commonly
boils down to solving classical decision problems for finite automata; usually automata
operating on infinite words. We study Parikh automata on finite and infinite words with applications to model checking. These automata extend classical finite automata with a Counting mechanism.
Still, due the dynamics of the realworld it might be the case that a change in the environmentleads to a faulty or even dangerous state of such a system. Referring to the above example of devices scanning for environmental threats, assume that due to a mistake at a local construction site the connection to the other devices has been cut, leaving a part of the country unmonitored. Hence, a new solution has to be computed. However, the new solution can differ arbitrarily from the current state. Given that it is costly to move such devices around, it is desirable to compute a new solution that is close to the current state. To capture and formalize such scenarios, we introduce the new framework of solution discovery via reconfiguration for constructing a feasible solution for a given problem by executing a sequence of small modifications starting from a given state
Homomorphism-Distinguishing Closedness for Graphs of Bounded Tree-Width
No full textTwo graphs are homomorphism indistinguishable over a graph class , denoted by G ≡_ H, if hom(F,G) = hom(F,H) for all F ∈ where hom(F,G) denotes the number of homomorphisms from F to G. A classical result of Lovász shows that isomorphism between graphs is equivalent to homomorphism indistinguishability over the class of all graphs. More recently, there has been a series of works giving natural algebraic and/or logical characterizations for homomorphism indistinguishability over certain restricted graph classes.
A class of graphs is homomorphism-distinguishing closed if, for every F ∉ , there are graphs G and H such that G ≡_ H and hom(F,G) ≠ hom(F,H). Roberson conjectured that every class closed under taking minors and disjoint unions is homomorphism-distinguishing closed which implies that every such class defines a distinct equivalence relation between graphs. In this work, we confirm this conjecture for the classes _k, k ≥ 1, containing all graphs of tree-width at most k.
As an application of this result, we also characterize which subgraph counts are detected by the k-dimensional Weisfeiler-Leman algorithm. This answers an open question from [Arvind et al., J. Comput. Syst. Sci., 2020].53:153:1
Studienreport Algorithmische Vorhersage und Mitbestimmung (AVuM) – eine qualitative Interviewstudie
No full textDieser Studienreport beschreibt das Projekt „Algorithmische Vorhersage und Mitbestimmung“ (AVuM), welches den Einsatz von algorithmischen Verfahren für das Risikomanagement („Predictive Risk Intelligence“ oder PRI) in zunehmend komplexen Wertschöpfungsnetzwerken untersucht. Neueste Modelle solcher PRI-Software sollen nun, unter anderem auf Basis von öffentlichen Daten und Daten von Social-Media-Plattformen, auch soziale Ereignisse wie Demonstrationen, Streiks und Kundgebungen vorhersagen können. Damit werden Fragen der betrieblichen und überbetrieblichen Mitbestimmung berührt.
Bisher ist nur sehr wenig darüber bekannt, wie Unternehmen PRI einsetzen, um Prozesse der Mitbestimmung zu beeinflussen. Daneben ist auch unklar, wie Gewerkschaften und Arbeitnehmer:innenvertretungen sich gegenüber PRI-Verfahren positionieren sollen und wie sie diese potenziell auch zu ihrem eigenen Vorteil einsetzen können. Dieses Projekt untersucht, wie und mit welchen Konsequenzen für die betriebliche und überbetriebliche Mitbestimmung Predictive Risk Intelligence (PRI) von Unternehmen bereits eingesetzt wird. Zudem wird untersucht, wie algorithmische Vorhersagesysteme von Arbeitnehmer:innenvertretungen genutzt werden können, um Mitbestimmung in Zeiten der Entsolidarisierung weiterzuentwickeln.
Hierzu wurden über einen Zeitraum von acht Monaten mit leitfadengestützten Experteninterviews einunddreißig Interviewpartner*innen aus drei Stakeholdergruppen befragt: 1. „Intermediaries“ (Vertreter aus Softwareentwicklung und -vertrieb von digitalen Risikomanagementtools), 2. „Audiences“ (Interessenvertretungen) und 3. „Companies“ (Kund:innen bzw. potenzielle Kund:innen von PRI-Verfahren). Schwerpunkte der Interviews waren die Einordnung der eigenen Organisation in den Kontext von globalen Lieferketten und Risikomanagement, Erfahrungen oder Einschätzungen zum Einsatz von PRI sowie die Beziehung zu anderen Stakeholdergruppen.Hans-Böckler-Stiftun
Hybrid functionals for periodic systems in the density functional tight-binding method
No full textOver the last decades, hybrid exchange-correlation functionals have been established as a standard technique to address the delocalization problem of semi-local Kohn-Sham density functional theory (KS-DFT). The opposing delocalization in DFT and over-localization in Hartree-Fock (HF) theory gives rise to mutual error compensation when admixing exact HF-type exchange to the density functional approximation (DFA). Despite the ever-growing computational resources and algorithmic improvements, non-local HF-type exchange remains an expensive tool within first principles frameworks, particularly for periodic systems.
Since its extension to purely long-range corrected hybrid functionals, the approximate density functional tight binding (DFTB) method, which is derived directly from KS-DFT, offers comparable quantum mechanical insights at a fraction of the computational cost. While the original formalism was restricted to molecules and long-range Fock exchange, this work generalizes the theoretical foundation and implementation to periodic boundary conditions beyond the Γ-point, covering the general class of range-separated hybrid functionals with Coulomb-attenuating method (CAM) type partitioning of the electron-electron interaction. Periodic boundary conditions are essential in broadening the applicability of hybrid DFTB, as most solid state systems cannot be adequately represented as cluster models. By implementing our work in the open-source software package DFTB+, a novel spectrum of methods allowing for an efficient treatment of problems beyond the reach of first principles schemes is made available to the materials science community.
For the first time, we demonstrate optimally tuned screened range-separated hybrid DFTB to provide a qualitatively correct description of the polarization-induced fundamental gap renormalization in molecular crystals. Dielectric-dependent global hybrid DFTB accurately reproduces measured band gaps of simple bulk materials, that cover the entire range from narrow- to wide-gap semiconductors as well as insulators. At the same level of theory, we obtain the phonon-induced band gap renormalization of prototypical indirect semiconductors over a wide temperature range. To sample the nuclear wave function, we employ Williams-Lax theory, evaluated either by stochastic Monte-Carlo integration or a deterministic one-shot procedure, as well as classical Born-Oppenheimer molecular dynamics. Following the trend of higher-level many-body perturbation theory, such as Hedin's GW approximation, HF-type exchange admixed to the DFA systematically yields a slightly stronger electron-phonon renormalization, including zero-point corrections
Lernsituationen entwickeln
No full textDie Leitidee der Arbeitsprozess- und Kompetenzorientierung in der Berufsbildung führt für Lehrkräfte an Berufsbildenden Schulen u. a. auch zu der Herausforderung, berufliche Lehr-/Lernprozesse entsprechend zu gestalten. Für den lernfeldorientierten Berufsschulunterricht erfolgt dies gemäß den Vorgaben der Kultusministerkonferenz (KMK) mithilfe von Lernsituationen.
Wie sich solche Lernsituationen entwickeln lassen, wird im vorliegenden Handbuch schrittweise gezeigt. Jeder Schritt wird ausführlich erklärt, die erforderlichen Dokumentationsvorlagen werden eingeführt, erläutert und in ihrer Anwendung durch Beispiele veranschaulicht
Influence of Atlantic Meridional Overturning Circulation changes on deglacial deep-water ecosystems off NW-Africa
No full textThe Atlantic Ocean is an important component of the global climate system. In this ocean basin, the Atlantic Meridional Overturning Circulation (AMOC), partially driven by the formation of North Atlantic Deep Water (NADW) in the northern hemisphere, uptakes an important percentage of the anthropogenically induced excess heat of our planet and distributes it around the globe through a complex ocean circulation system that influences global warming rate (e.g. Cummins et al., 2016; Levitus et al., 2000). As described in Chapter 2 and Chapter 4, the instrumental record of AMOC strength provides evidence of a potential decrease or weakening also known as “AMOC slowdown” in the 19th and 20th centuries, synchronous with the increasing trend of CO2 in the atmosphere, as well as greenhouse gases concentrations (Caesar et al., 2021; Dima et al., 2021; IPCC, 2022; Rahmstorf et al., 2015). Furthermore, AMOC will very likely decrease in the 21st century (IPCC, 2023). However, the instrumental AMOC record is still too short to fully understand the effects of such slowdown in the climate system, therefore our knowledge must rely on paleoceanographic proxies. Although controversial, AMOC slowdown has been recorded by different proxies in the last 30,000 years, offering the opportunity to observe the effects of this slowdown in the deep ocean through the marine fossil record. This is discussed in detail in the Introduction of Chapter 4.
The last deglaciation period in the Eastern North Atlantic (NE Atlantic) starts after the end of the Last Glacial Maximum (LGM, ⁓ 19 kyrs BP). Temporary returns to cold periods occurred during the Heinrich Stadial 1 (HS1, ⁓18.3 – 15.4 Kyr BP) and Younger Dryas (YD, ⁓12.9 – 11.7 Kyr BP), and between these two cold periods, the Bølling–Allerød warming (B-A) was a transient warming time in the northern hemisphere. The record of AMOC, includes kinematic proxies like 231Pa/230Th (Böhm et al., 2015; Gherardi et al., 2005; McManus et al., 2004), paleo-circulation proxies like the radiogenic neodymium (Nd) isotope composition (Böhm et al., 2015; Du et al., 2020; Roberts et al., 2010) and bottom water ventilation proxies like the benthic foraminifera δ13C (e.g., Oppo et al., 2015), that show a consistent AMOC decline during the HS1 and YD, alternated by a short B-A resumption period.
In this research, deglacial sedimentary records of gravity cores GeoB9512-5 (15.34 °N, 17.37 °W, 793 m water depth), GeoB9508-5 (15.50 °N, 17.95 °W, 2,384 m WD) and GeoB9506-1 (15.61 °N, 18.35 °W, 2,956 m WD) were analyzed to extract and investigate the benthic foraminifera content, and reconstruct the bottom water changes in the NE Atlantic during the last 27,000 years. Three components of bottom waters were investigated; (1) bottom water oxygenation reconstructed from the detailed taxonomic and quantitative analyses of benthic foraminifera (GeoB9512-5 and GeoB9506-1); (2) bottom water temperatures (BWTs) reconstructed from benthic foraminifera Magnesium-Calcium (Mg/Ca) ratios (GeoB9512-5, GeoB9508-5 and GeoB9506-1); and (3) changes in ventilation and salinity inferred from benthic foraminifera δ13C and δ18O respectively (GeoB9512-5, GeoB9508-5 and GeoB9506-1). The key outcomes of this research are: (1) deep-ocean heat uptake stagnation in times of reduced AMOC in the NE Atlantic, suggesting AMOC strength sets the oceanic heat storage depth; (2) a better ventilated and oxygenated deglacial Eastern Tropical North Atlantic Oxygen Minimum Zone, related to an intensified subtropical cell due to AMOC slowdown; than in turn (3) affected calcification processes of some benthic foraminifera at intermediate depths introducing uncertainties on Mg/Ca-based paleotemperature reconstructions at intermediate depths in the eastern North Atlantic. Finally, the integrated paleotemperature records show that during the LGM and most of the last deglacial period, heat storage took place in the intermediate waters in the tropical eastern Atlantic, and shift to a deep mode in the Holocene as AMOC strength is restored
Remote sensing of sea ice leads with Sentinel-1 C-band synthetic aperture radar
No full textThe presence of leads with open water or thin ice is an important feature of the Arctic sea ice cover.
Leads regulate the heat, gas, and moisture fluxes between the ocean and atmosphere and are areas of high ice growth
rates during periods of freezing conditions.
In the present study an algorithm providing an automatic lead detection based on Synthetic Aperture Radar (SAR) images
is developed using traditional machine learning techniques and deep learning methods.
The algorithm is applied to a wide range of Sentinel-1 scenes taken over the Arctic Ocean.
Distribution of the detected leads in the Arctic during winter seasons 2016--2021 is then analyzed.
An important part of the algorithm development is the data preprocessing as the classification quality depends
on the quality of the input images.
An advanced data preparation technique improves consistency of the cross-polarization channel and enables the use of dual-polarization SAR images.
By using both the HH and the HV channels instead of single co-polarized observations the algorithm is able to detect
more leads compared to the use of the HH polarization only.
First, a traditional machine learning approach is described.
It is based on polarimetric features and texture features derived from the grey level co-occurrence matrix.
The Random Forest classifier is used to investigate the individual feature importance on the lead detection.
The precision-recall curve representing the quality of the classification is assessed to define a threshold for the binary
lead/sea ice classification.
The algorithm produces a lead classification with more than 90% precision with 60% of all leads classified, as evaluated on the test data.
The precision can be increased by the cost of the amount of leads detected.
Classification quality is improved by introducing an advanced binarization method based on watershed segmentation.
Further improvements include object shape analysis resulting in a shape-based filter, which efficiently removes
objects appearing due to noise patterns over young ice.
Second, an algorithm based on a convolutional neural network is developed.
It shows more robust results compared to the algorithm based on the gray level co-occurrence matrix with Random Forest classification
and is applicable to the entire Arctic Ocean.
Classification results are evaluated against the dataset which does not include training or testing data,
and are also compared to Sentinel-2 optical satellite images.
Finally, the lead detection algorithm is applied to all Sentinel-1 EW GRDM scenes taken in five winter seasons,
1 November - 30 April of 2016-2021 years.
3-day composite pan-Arctic lead maps with the native Sentinel-1 40~meters pixel spacing are produces.
The frequency of lead occurrence derived from these maps is compared with MODIS thermal infrared lead detection results.
The lead area fraction is compared with the AMSR2 passive microwave observations.
The lead area distribution, lead length, and lead width distributions, as well as the lead orientation distributions, are analyzed
in the following regions of the Arctic Ocean: Fram Strait, Barents Sea, Kara Sea, Laptev Sea, East Siberian Sea,
Chukchi Sea, Beaufort Sea, Central Arctic.
Each region shows the presence of regularity in lead orientation, the preferred orientation has little variation from year to year and during season.
The lead width distribution is found to follow the power low with the exponent of 1.86 with 0.16 standard deviation.
The yearly mean lead area fraction derived from Sentinel-1 images varies from 2.5% to 3.7% during winter seasons 2016-2021
Was da kreucht und fleucht : Tierdarstellungen in spätmittelalterlichen Handschriften der SuUB Bremen : Begleitband zur Ausstellung
No full textTiere waren im Mittelalter ständiger Begleiter des Menschen. So wundert es nicht, dass ihnen in den mittelalterlichen Buchhandschriften der Staats- und Universitätsbibliothek Bremen eine bedeutende Rolle zukommt. Farbenfrohe Drachen, Löwen, Schlangen, Greife, einheimische Tiere wie Hunde oder Pferde, biblische Tiere wie der Esel und schließlich Mischwesen bewohnen die Miniaturen, die Initialen und die Randbereiche der Codices. Der vorliegende Band befasst sich mit diesen Tierdarstellungen in mittelalterlichen Handschriften der SuUB Bremen aus historischer, kultur- und kunsthistorischer sowie restauratorischer Perspektive.1
Fluiddynamische Beeinflussung der laserchemischen Mikrostrukturierung von Metallen
No full textAufgrund des laserinduzierten Elektrolytsiedeprozesses und des folgenden
abschirmenden Effekts der Gasblasen werden die Obergrenzen der Abtragsraten während der laserchemischen Bearbeitung stark eingeschränkt, woraus ein Hemmnis für die industrielle Anwendung resultiert. In dieser Arbeit wurde das Ziel verfolgt, den dominierenden Wirkmechanismus der Gasblasenabschirmung zu identifizieren und die Obergrenzen der Abtragsraten zu steigern.Wirkmechanismus des abschirmenden Effekts der Gasblasen identifiziert werden. Durch fluiddynamische Beeinflussungen, wie die Erhöhung des Prozessdrucks und die Reduktion der Elektrolytviskosität konnte der abschirmende Effekt der Gasblasen reduziert werden. Dadurch konnten die Obergrenzen der Abtragsraten während der laserchemischen Bearbeitung gesteigert werden. Abschließend wurde eine Möglichkeit zur Vorhersage der laserchemischen Abtragskategorie aufgezeigt
The transitional regime of pulsatile pipe flow
No full textThe presence of turbulence in the circulatory system is thought to lead to cardiovascular diseases. Despite its importance, turbulence transition in cardiovascular flows is not well understood. In particular, it is unclear which one of the numerous complex features of blood flow (unsteady driving, rheology, flexible walls, complex geometry...) is the dominant one in terms of turbulence transition. The main aim of this thesis is to single out the effects of one of these features: the unsteady driving of the flow.
Specifically, the case of a pulsatile driven, Newtonian fluid, in a rigid smooth pipe of circular cross-section is considered, referred to as pulsatile pipe flow. Two main questions are investigated: whether and how laminar pulsatile pipe flows transition to turbulence, and how turbulence behaves once triggered. Pulsatile pipe flows in the transitional regime, with a mean 1000<Re<3000, are considered. Apart from single harmonic pulsations, different waveforms are considered, including waveforms relevant for physiological flows.
By combining linear transient growth and stability analyses, it is demonstrated that, at intermediate pulsation frequencies (4<Wo<20) and moderate to high pulsation amplitudes (0.5<A<3), the laminar pulsatile pipe flow is highly susceptible to large disturbance amplification. Coincidentally the blood flow in the human aorta falls in this parameter regime. The underlying mechanisms related to this susceptibility are identified, and their dependence with respect to the flow parameters explored. Additionally, it is shown that, specific features of the driving waveform can enhance these mechanisms. In particular, bulk velocities with steep acceleration/deceleration phases and, counter-intuitively, with longer low velocity phases promote turbulence transition.
The turbulence behavior in this broad parametric space is studied with the use of a large number of direct numerical simulations. As part of this thesis a new C-CUDA code was developed in order to perform fast direct numerical simulations. The code outperforms state-of-the-art CPU codes in terms of computing time and computing resources. With the use of a causal analysis, it is shown that turbulence production increases due to the same mechanisms that render the flow susceptible to transition. Finally, a reduced-order model is developed to approximate the behavior of turbulence in pulsatile pipe flow reasonably well.
In sum, this thesis describes the way the flow is more likely to transition to turbulence in this
parametric regime, and the behavior of turbulence once triggered. The results presented here suggest that blood flow in the larger arteries is susceptible to transition due to the pulsatile beating of our hearts alone