Computing Sacker-Sell spectra in discrete time dynamical systems

In this paper we develop two boundary value methods for detecting Sacker-Sell spectra in discrete time dynamical systems. The algorithms are advancements of earlier methods for computing projectors of exponential dichotomies. The first method is based on the projector residual PP − P. If this residual is large, then the difference equation has no exponential dichotomy. A second criterion for detecting Sacker-Sell spectral intervals is the norm of end points of the solution of a specific boundary value problem. Refined error estimates for the underlying approximation process are given and the resulting algorithms are applied to an example with known continuous Sacker-Sell spectrum, as well as to the variational equation along orbits of Hénon’s map

Computing covariant vectors, Lyapunov vectors, Oseledets vectors, and dichotomy projectors: a comparative numerical study

Covariant vectors, Lyapunov vectors, or Oseledets vectors are increasingly being used for a variety of model analyses in areas such as partial differential equations, nonautonomous differentiable dynamical systems, and random dynamical systems. These vectors identify spatially varying directions of specific asymptotic growth rates and obey equivariance principles. In recent years new computational methods for approximating Oseledets vectors have been developed, motivated by increasing model complexity and greater demands for accuracy. In this numerical study we introduce two new approaches based on singular value decomposition and exponential dichotomies and comparatively review and improve two recent popular approaches of Ginelli et al. (2007) and Wolfe and Samelson (2007). We compare the performance of the four approaches via three case studies with very different dynamics in terms of symmetry, spectral separation, and dimension. We also investigate which methods perform well with limited data

AMPA-receptor-mediated excitatory synaptic transmission is enhanced by iron-induced α-synuclein oligomers

Die α-Synuclein-Aggregation ist ein charakteristisches pathologisches Schlüsselereignis bei neurodegenerativen Erkrankungen, wie Morbus Parkinson (MP) und Demenz mit Lewy- Körperchen (DLB). Neuere Untersuchungen weisen darauf hin, dass α-Synuclein-Oligomere die wesentliche neurotoxische Spezies darstellen. Der zugrundeliegende Mechanismus konnte dabei noch nicht vollständig geklärt werden. In der vorliegenden Arbeit wurde der Effekt verschiedener α-Synuclein-Oligomer-Präparationen auf die synaptische Übertragung in autaptischen neuronalen Kulturen untersucht. Applikationen von großen α-Synuclein- Oligomeren erhöhten selektiv die Amplitude von evozierten AMPA-Rezeptor-vermittelten synaptischen Strömen innerhalb von Minuten. Die Applikation von kleinen Oligomere erhöhte die Amplitude hingegen nicht. Die Amplituden der NMDA-Rezeptor-vermittelten synaptischen Ströme wurden von keiner der beiden Oligomerspezies beeinflusst. Die Biotinylierung von AMPA-Rezeptoren in akuten Hirnschnitten zeigte eine gesteigerte Rezeptorinsertion. Dies deutet auf einen postsynaptischen Mechanismus hin, bei dem die Ca2+-Konzentration erhöht wird. Weiterhin konnte bei den großen α-Synuclein-Oligomeren eine Veränderung an der Präsynapse beobachtet werden. Sowohl die Frequenz der spontanen postsynaptischen Ströme (sEPSCs) in kultivierten Neuronen als auch die synaptische Vesikelausschüttung in Synaptosomenpräparationen wurden nach Applikation großer α- Synuclein-Oligomere erhöht. Um zu untersuchen, ob α-Synuclein-Oligomere, induziert durch die synaptische Transmission, einen Effekt auf das neuronale Überleben hatten, wurden MTT-Assays durchgeführt. Es konnte ein Anstieg der Glutamat-Toxizität in Gegenwart von großen α-Synuclein-Oligomeren gezeigt werden, was auf einen exzitatorischen Mechanismus im neuronalen Überleben hindeutet.α-Synuclein aggregation is a characteristic pathological finding in neurogenerative diseases including Parkinson ́s disease and dementia with Lewy bodies. Recent findings suggest that α-synuclein oligomers represent the principal neurotoxic species however their mode of action is not fully understood. Here we studied the effect of various well-defined oligomeric α-synuclein preparations on synaptic transmission in autaptic neuronal cultures. Preparations leading to large oligomers but not those leading to small ones were found to enhance selectivity of the amplitude of evoked AMPA receptor but not NMDA receptor-mediated synaptic currents within minutes. AMPA receptor biotinylation studies in acute brain slices reveal a postsynaptic mechanism namely an enhanced receptor insertion. In addition we obtained evidence that large synuclein oligomers do have a presynaptic action as well: Both the frequency of spontaneous miniature synaptic current in cultured neurons as well as the synaptic vesicle release in synaptosomal preparations were enhanced following the application of α -synuclein oligomers. In order to work out if α-synuclein oligomer induced by enhanced excitatory synaptic transmission affect neuronal survival MTT assays were performed. Here we observed enhanced glutamate toxicity in the presence of large α- synuclein oligomers indicating that α-synuclein oligomers do alter neuronal survival via an excitotoxic mechanism

Performance of absorption chillers in field tests

Absorption chillers can use waste heat, solar heat, or excess heat by cogeneration facilities to supply chilled water. Therefore, absorption chillers are important components for poly-generation, which can improve plant utilization and efficiency of the overall energy supply system. Currently, 27 new small and medium-scale (30–160 kW cooling capacity) absorption chillers with high efficiency in full- and part-load as well as low specific size and weight are the focus of two research projects sponsored by German Federal Ministries. The chillers are located in 20 different sites, four of them in Jordan and the rest in Germany. The main objective of these projects is to enhance trigeneration and solar cooling system efficiency. The Jordan projects and more than 70% of the German installations use dry cooling towers. Combined heat and power (CHP) plants or district heating grids provide the driving heat for regeneration at the German installations. Solar thermal collectors drive one German and all Jordan installations. Within the projects, the chillers are provided with an intelligent control algorithm that allows achieving several objectives at the same time. One of the objectives, of course, is to match the desired chilled water temperature as well as the cooling capacity. Another objective is, for instance, to fit the hot water outlet temperature. This is commonly essential in cogeneration to avoid emergency shutdowns of the CHP engine. The field operation shows that the deviation of said temperatures from the set point is usually less than ±0.5 K in operation. In those operational hours in which the temperatures of the heat source or heat sink do not allow to reach all objectives, a set of hierarchical aims can be established within the algorithm so that the chiller can be controlled to match its prioritized objectives. Furthermore, the designed absorption chiller performs dynamically and autonomously in a very large range of temperatures and flow rates. These can vary from 20% up to 150% of nominal conditions. This flexibility can be used to achieve up to 80% savings of electrical power consumption using speed-controlled pumps in part load conditions. In contrast to the common opinion that small and medium-scale absorption chillers are not competitive as compared to compression chillers, this paper shows applications in which absorption chillers are advantageous because of synergies with and multiple benefits within the rest of the energy supply system

Millennial-scale SST variability as inferred from planktonic foraminiferal census counts in the western subtropical Atlantic

Volume: 1915Start Page: 256End Page: 26