On the representation dimension of skew group algebras, wreath products and blocks of Hecke algebras

We establish bounds for the representation dimension of skew group algebras and wreath products. Using this, we obtain bounds for the representation dimension of a block of a Hecke algebra of type A, in terms of the weight of the block. This includes certain blocks of group algebras of symmetric groups.Comment: 8 page

Damage mechanisms in superconductors due to the impact of high energy proton beams and radiation tolerance of cryogenic diodes used in particle accelerator magnet systems

High energy hadron accelerators such as the Large Hadron Collider (LHC) at CERN and its planned upgrade to achieve higher luminosity, the High Luminosity Large Hadron Collider (HL-LHC), require superconducting magnets to provide strong magnetic fields, needed to steer and focus the particle beams at these high energies. During operation the superconducting magnets and their components are exposed to radiation resulting from primary and secondary particles from two main sources of beam losses. During normal operation, steady state losses resulting from interaction of the particle beams with residual gas molecules or from particle debris in interaction points affect the accelerator magnets and their components along the machine. In case of failures, significant parts of the beam can be lost in a short time, resulting in localized damage due to heating from energy deposition, which in turn causes thermo-mechanical stresses and strains. In the HL-LHC, novel focusing superconducting quadrupole magnets will be installed, based on Nb$_3$Sn and located close to the interaction points. Furthermore, the beam intensity will be doubled. Both, steady state losses and the severity of losses due to fast failures scale with the beam intensity. In this thesis, effects of beam losses on accelerator magnet components were studied. Firstly, the effects of high intensity and high energy proton beam impact on the low temperature superconductors (LTS) Nb-Ti, Nb$_3$Sn and tapes based on the high temperature superconductor (HTS) YBCO were studied. An experiment was performed where beam was directed on superconductors in a cryogenic environment in CERN’s HiRadMat facility. The performance of the superconductors was afterwards analyzed for their critical transport current, critical field and temperature, as well as inspected with optical and electron microscopic methods. The experimental setup, the observed damage mechanisms and the subsequent analysis are discussed. Secondly, the powering layout of the magnet circuits foresees the use of cryogenic power diodes, connected in parallel to each magnet, serving as passive protection in case of a quench. The diodes are located in close proximity to the beam axis and are affected by the enhanced radiation levels close to the interaction points. To identify a diode type that can be safely operated during the lifetime of HL-LHC, the radiation hardness of existing LHC-type diodes and prototype diodes, that are expected to be more radiation tolerant were tested. An experiment was set up, which allowed the irradiation and in situ measurements of three different types of diodes at cryogenic temperatures. All prototypes were analyzed for forward and reverse bias voltage characteristics and the temperature dependence while warming up. Their thermal annealing potential could also be evaluated. The experimental setup, the in situ measurements and the subsequent analysis are discussed

The structural sheath protein of aphids is required for phloem feeding

AbstractAphids produce two types of saliva that mediate their interactions with plants. Watery saliva is secreted during cell penetration and ingestion, whereas gel saliva is secreted during stylet movement through the apoplast where it forms a sheath around the stylet to facilitate penetration and seal puncture sites on cell membranes. In order to study the function of the sheath when aphids interact with plants, we used RNA interference (RNAi) to silence the aphid structural sheath protein (SHP) in the pea aphid Acyrthosiphon pisum. The injection of 50 ng of double stranded RNA completely disrupted sheath formation, as confirmed by scanning electron microscopy. Aphid behavior was monitored using the electrical penetration graph technique, revealing that disrupted sheath formation prevented efficient long-term feeding from sieve tubes, with a silencing effect on reproduction but not survival. We propose that sealing the stylet penetration site in the sieve tube plasma membrane is part of a two-step mechanism to suppress sieve-tube occlusion by preventing calcium influx into the sieve tube lumen. The SHP is present in several aphid species and silencing has a similar impact to aphid-resistant plants, suggesting that SHP is an excellent target for RNAi-mediated pest control

Fast Algorithms at Low Temperatures via Markov Chains

For spin systems, such as the hard-core model on independent sets weighted by fugacity lambda>0, efficient algorithms for the associated approximate counting/sampling problems typically apply in the high-temperature region, corresponding to low fugacity. Recent work of Jenssen, Keevash and Perkins (2019) yields an FPTAS for approximating the partition function (and an efficient sampling algorithm) on bounded-degree (bipartite) expander graphs for the hard-core model at sufficiently high fugacity, and also the ferromagnetic Potts model at sufficiently low temperatures. Their method is based on using the cluster expansion to obtain a complex zero-free region for the partition function of a polymer model, and then approximating this partition function using the polynomial interpolation method of Barvinok. We present a simple discrete-time Markov chain for abstract polymer models, and present an elementary proof of rapid mixing of this new chain under sufficient decay of the polymer weights. Applying these general polymer results to the hard-core and ferromagnetic Potts models on bounded-degree (bipartite) expander graphs yields fast algorithms with running time O(n log n) for the Potts model and O(n^2 log n) for the hard-core model, in contrast to typical running times of n^{O(log Delta)} for algorithms based on Barvinok\u27s polynomial interpolation method on graphs of maximum degree Delta. In addition, our approach via our polymer model Markov chain is conceptually simpler as it circumvents the zero-free analysis and the generalization to complex parameters. Finally, we combine our results for the hard-core and ferromagnetic Potts models with standard Markov chain comparison tools to obtain polynomial mixing time for the usual spin system Glauber dynamics restricted to even and odd or "red" dominant portions of the respective state spaces

Rigor and Relevance: Enforcing dialogue between media management research and practice

There is a strong demand in media management to strengthen the bridge between theory and practice. Following the debate on rigor and relevance, the approach of engaged scholarship and arguments from our community, we see the need to develop a research agenda in dialogue with practice. Therefore, we explore the following questions: Which topics do media managers consider relevant to research? How can these issues be contrasted with the research agendas proclaimed by media management research? To answer these questions, we first summarize current issues in media management research through a literature review. Second, current issues in media management practice are captured based on an online survey with German media managers (N=46). Finally, the two perspectives are contrasted to develop relevant questions for media management research and practice

Perfect sampling for hard spheres from strong spatial mixing

We provide a perfect sampling algorithm for the hard-sphere model on subsets of R^d with expected running time linear in the volume under the assumption of strong spatial mixing. A large number of perfect and approximate sampling algorithms have been devised to sample from the hard-sphere model, and our perfect sampling algorithm is efficient for a range of parameters for which only efficient approximate samplers were previously known and is faster than these known approximate approaches. Our methods also extend to the more general setting of Gibbs point processes interacting via finite-range, repulsive potentials

Influence of Sodium Chloride on the Formation and Dissociation Behavior of CO2 Gas Hydrates

We present an experimental study on the formation and dissociation characteristics of carbon dioxide (CO2) gas hydrates using Raman spectroscopy. The CO2 hydrates were formed from sodium chloride/water solutions with salinities of 0–10 wt %, which were pressurized with liquid CO2 in a stirred vessel at 6 MPa and a subcooling of 9.5 K. The formation of the CO2 hydrate resulted in a hydrate gel where the solid hydrate can be considered as the continuous phase that includes small amounts of a dispersed liquid water-rich phase that has not been converted to hydrate. During the hydrate formation process we quantified the fraction of solid hydrate, xH, and the fraction of the dispersed liquid water-rich phase, xL, from the signature of the hydroxyl (OH)-stretching vibration of the hydrate gel. We found that the fraction of hydrate xH contained in the hydrate gel linearly depends on the salinity of the initial liquid water-rich phase. In addition, the ratio of CO2 and water was analyzed in the liquid water-rich phase before hydrate formation, in the hydrate gel during growth and dissociation, and after its complete dissociation again in the liquid water-rich phase. We observed a supersaturation of CO2 in the water-rich phase after complete dissociation of the hydrate gel and were able to show that the excess CO2 exists as dispersed micro- or nanoscale liquid droplets in the liquid water-rich phase. These residual nano- and microdroplets could be a possible explanation for the so-called memory effect

Diffusion und Adoption neuer Formen des Home Entertainments. Ein Überblick der Nutzerstrukturen von PVR und DVD-Rekorder Systemen

Während sich im Bereich des Home Entertainments die DVD als digitales Abspielformat weitgehend durchgesetzt hat, besteht Unklarheit darüber, welche Form der digitalen Aufzeichnung (PVR, DVD-Rekorder) sich in den nächsten Jahren durchsetzen wird. Der vorliegende Bericht untersucht die Nutzerstrukturen solcher neuer, innovativer Aufnahmegeräte im Bereich des Home Entertainment. Der Bericht ist in der Reihe "Menschen, Märkte, Medien, Management - Berichte aus Forschung und Lehre" des Fachgebiets Medienmanagement erschiene

A Web-Based Multi-Screen 360-Degree Video Player For Pre-Service Teacher Training

This demonstration will showcase a new and innovative eLearning platform for pre-service teacher training. The core element of this platform is a multi-screen 360-degree video player with additional features for 360-degree video analysis. By using the videos in combination with a head-mounted display (HMD) we create a video-based virtual classroom, where the pre-service teachers “become part of the situation”. This offers students an immersive experience to get a first impression of realistic school praxis

A Web-Based Multi-Screen 360-Degree Video Player For Pre-Service Teacher Training

This demonstration will showcase a new and innovative eLearning platform for pre-service teacher training. The core element of this platform is a multi-screen 360-degree video player with additional features for 360-degree video analysis. By using the videos in combination with a head-mounted display (HMD) we create a video-based virtual classroom, where the pre-service teachers “become part of the situation”. This offers students an immersive experience to get a first impression of realistic school praxis