Physics prospects of UV-filtered overlap quarks

Some key features of the overlap operator with a UV-filtered Wilson kernel are discussed. The first part concerns spectral properties of the underlying shifted hermitean Wilson operator and the relation to the observed speedup of the overlap construction. Next, the localization of the filtered overlap and its axial-vector renormalization constant are discussed. Finally, results of an exploratory scaling study for $m_{ud}, m_s$ and $f_\pi, f_K$ are presented.Comment: Talk given at Workshop on Computational Hadron Physics, Nicosia, Cyprus, 14-17 Sep 200

Shape: A 3D Modeling Tool for Astrophysics

We present a flexible interactive 3D morpho-kinematical modeling application for astrophysics. Compared to other systems, our application reduces the restrictions on the physical assumptions, data type and amount that is required for a reconstruction of an object's morphology. It is one of the first publicly available tools to apply interactive graphics to astrophysical modeling. The tool allows astrophysicists to provide a-priori knowledge about the object by interactively defining 3D structural elements. By direct comparison of model prediction with observational data, model parameters can then be automatically optimized to fit the observation. The tool has already been successfully used in a number of astrophysical research projects.Comment: 13 pages, 11 figures, accepted for publication in the "IEEE Transactions on Visualization and Computer Graphics

Isoperimetric inequalities vs. upper curvature bounds

The Dehn function of a metric space measures the area necessary in order to fill a closed curve of controlled length by a disc. As a main result, we prove that a length space has curvature bounded above by $\kappa$ in the sense of Alexandrov if and only if its Dehn function is bounded above by the Dehn function of the model surface of constant curvature $\kappa$. This extends work of Lytchak and the second author from locally compact spaces to the general case. A key ingredient in the proof is the construction of minimal discs with suitable properties in certain ultralimits. Our arguments also yield quantitative local and stable versions of our main result. The latter has implications on the geometry of asymptotic cones

Regularisierte Optimierungsverfahren für Rekonstruktion und Modellierung in der Computergraphik

The field of computer graphics deals with virtual representations of the real world. These can be obtained either through reconstruction of a model from measurements, or by directly modeling a virtual object, often on a real-world example. The former is often formalized as a regularized optimization problem, in which a data term ensures consistency between model and data and a regularization term promotes solutions that have high a priori probability. In this dissertation, different reconstruction problems in computer graphics are shown to be instances of a common class of optimization problems which can be solved using a uniform algorithmic framework. Moreover, it is shown that similar optimization methods can also be used to solve data-based modeling problems, where the amount of information that can be obtained from measurements is insufficient for accurate reconstruction. As real-world examples of reconstruction problems, sparsity and group sparsity methods are presented for radio interferometric image reconstruction in static and time-dependent settings. As a modeling example, analogous approaches are investigated to automatically create volumetric models of astronomical nebulae from single images based on symmetry assumptions.Das Feld der Computergraphik beschäftigt sich mit virtuellen Abbildern der realen Welt. Diese können erlangt werden durch Rekonstruktion eines Modells aus Messdaten, oder durch direkte Modellierung eines virtuellen Objekts, oft nach einem realen Vorbild. Ersteres wird oft als regularisiertes Optimierungsproblem dargestellt, in dem ein Datenterm die Konsistenz zwischen Modell und Daten sicherstellt, während ein Regularisierungsterm Lösungen fördert, die eine hohe A-priori-Wahrscheinlichkeit aufweisen. In dieser Arbeit wird gezeigt, dass verschiedene Rekonstruktionsprobleme der Computergraphik Instanzen einer gemeinsamen Klasse von Optimierungsproblemen sind, die mit einem einheitlichen algorithmischen Framework gelöst werden können. Darüber hinaus wird gezeigt, dass vergleichbare Optimierungsverfahren auch genutzt werden können, um Probleme der datenbasierten Modellierung zu lösen, bei denen die aus Messungen verfügbaren Daten nicht für eine genaue Rekonstruktion ausreichen. Als praxisrelevante Beispiele für Rekonstruktionsprobleme werden Sparsity- und Group-Sparsity-Methoden für die radiointerferometrische Bildrekonstruktion im statischen und zeitabhängigen Fall vorgestellt. Als Beispiel für Modellierung werden analoge Verfahren untersucht, um basierend auf Symmetrieannahmen automatisch volumetrische Modelle astronomischer Nebel aus Einzelbildern zu erzeugen

Deep-Red Luminescent Molybdenum(0) Complexes with Bi- and Tridentate Isocyanide Chelate Ligands

In octahedral complexes, molybdenum(0) has the same 4cr valence electron configuration as ruthenium(II), which is beneficial for establishing energetically low-lying metal-to-ligand charge transfer (MLCT) excited states. Those MLCT states often show luminescence, and they can furthermore undergo photoinduced electron and energy transfer reactions that are of interest in the context of solar energy conversion, sensing, or photocatalysis. Molybdenum is roughly 100 times more abundant than ruthenium, and it seems desirable to increase our fundamental understanding of the photophysical properties of complexes made from non-precious metals. We report here on the luminescence behavior of two new homoleptic molybdenum(0) isocyanide complexes, one with three bidentate, the other with two tridentate chelate ligands. The key novelty is the incorporation of thiophene units into the ligand backbones, causing strongly red-shifted photoluminescence with respect to comparable molybdenum(0) isocyanides with phenylene units in the ligand backbones. Combined experimental and computational studies provide detailed insight into the photophysical properties of this compound class. This work is relevant for the development of new luminescent compounds with possible applications in lighting and sensing, and it complements current research efforts on photoactive complexes with other abundant transition metal and main group elements

A Sparse Reconstruction Algorithm for Multi-Frequency Radio Images

In radio interferometry, every pair of antennas in an array defines one sampling point in the Fourier domain of the sky image. By combining information from different wavelengths, sample coverage - and therefore reconstruction quality - can be increased. However, the images at different wavelengths can be dramatically dissimilar; this fact must be taken into account when reconstructing multi-frequency images. In this paper, we present a novel reconstruction algorithm based on the assumption that the spectrum is continuous. In contrast to prior work, we allow for sparse deviations from this assumption: this allows, for example, for accurate reconstruction of line spectra superimposed on a continuum. Using simulated measurements on synthetic multi-frequency images, we show that the proposed approach provides significant improvements over a comparable method based solely on a continuity assumption

SparseRI: A Compressed Sensing Framework for Aperture Synthesis Imaging in Radio Astronomy

In radio interferometry, information about a small region of the sky is obtained in the form of samples in the Fourier transform domain of the desired image. Since this sampling is usually incomplete, the missing information has to be reconstructed using additional assumptions about the image. The emerging field of Compressed Sensing (CS) provides a promising new approach to this type of problem which is based on the supposed sparsity of natural images in some transform domain. We present a versatile CS-based image reconstruction framework called SparseRI, an interesting alternative to the clean algorithm, that permits a wide choice of different regularisers for interferometric image reconstruction. The performance of our method is evaluated on simulated data as well as on actual radio interferometry measurements from the VLA, showing that our algorithm is able to reproduce the main features of the test sources. The proposed method is a first step towards an alternative reconstruction approach that may be able to avoid typical artefacts like negative flux regions, work with large fields of view and non-coplanar baselines, avoid the gridding process, and reduce the amount of manual work that is required in order to obtain best-quality results