###
new algorithm to *compute* fusion coecients By

"... This is a proceedings article reviewing a recent combinatorial construction of the bsu(n)k WZNW fusion ring by C. Stroppel and the author. It contains one novel aspect: the explicit derivation of an algorithm for the computation of fusion coecients dierent from the Kac-Walton formula. The discussion ..."

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This is a

*proceedings*article reviewing a recent combinatorial construction of the bsu(n)k WZNW fusion ring by*C*. Stroppel and the author. It contains one novel aspect: the explicit derivation of an algorithm for the*computation*of fusion coecients dierent from the Kac-Walton formula### COSMOLOGICAL MODELS OF MODIFIED GRAVITY

, 2013

"... The recent discovery of dark energy has prompted an investigation of ways in which the accelerated expansion of the universe can be realized. In this dissertation, we present two separate projects related to dark energy. The first project analyzes a class of braneworld models in which multiple brane ..."

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branes float in a five-dimensional anti-de Sitter bulk, while the

*second*investigates a class of dark energy models from an effective field theory perspective. Investigations of models including extra dimensions have led to modifications of gravity involving a number of interesting features### 1Local Identification of Overcomplete Dictionaries

"... This paper presents the first theoretical results showing that stable identification of overcomplete µ-coherent dictionaries Φ ∈ Rd×K is locally possible from training signals with sparsity levels S up to the order O(µ−2) and signal to noise ratios up to O( d). In particular the dictionary is recove ..."

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This paper presents the first theoretical results showing that stable identification of overcomplete µ-coherent dictionaries Φ ∈ Rd×K is locally possible from training signals with sparsity levels S up to the order O(µ−

*2*) and signal to noise ratios up to O( d). In particular the dictionary### 0 QUANTITATIVE UNIFORM IN TIME CHAOS PROPAGATION FOR BOLTZMANN COLLISION PROCESSES

, 2010

"... ar ..."

### A Dyson-Schwinger study of the four-gluon vertex

"... Abstract: We present a self-consistent calculation of the four-gluon vertex of Landau gauge Yang-Mills theory from a truncated Dyson-Schwinger equation. The equation contains the leading diagrams in the ultraviolet and is solved using as the only input results for lower Green functions from previous ..."

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Abstract: We present a self-consistent calculation of the four-gluon vertex of Landau gauge Yang-Mills theory from a truncated Dyson-Schwinger equation. The equation contains the leading diagrams in the ultraviolet and is solved using as the only input results for lower Green functions from previous Dyson-Schwinger calculations that are in good agreement with lattice data. All quantities are therefore fixed and no higher Green functions enter within this truncation. Our self-consistent solution resolves the full momentum dependence of the vertex but is limited to the tree-level tensor structure at the moment. Calculations of selected dressing functions for other tensor structures from this solution are used to exemplify that they are suppressed compared to the tree-level structure except for possible logarithmic enhancements in the deep infrared. Our results furthermore allow to extract a qualitative fit for the vertex and a running coupling.

### The Spherical Landau Problem

, 2001

"... The magnetization for electrons on a two-dimensional sphere, under a spherically symmetrical normal magnetic field has been studied in the large field limit. This allows us to use an Euclidean approximation for low energies electron states getting an analytical solution for the problem and avoiding ..."

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the difficulties of quantization on a curved manifold. At low temperatures our results are exact and allow direct comparisson with the planar Landau case. In this temperature limit we

*compute*the magnetization and show it exhibit an oscillatory de Hass-Van Alphen type of behaviour.### Abstract Cosmological Black Holes as Models of Cosmological Inhomogeneities

, 2006

"... Since cosmological black holes modify the density and pressure of the surrounding universe, and introduce heat conduction, they produce simple models of cosmologi-cal inhomogeneities that can be used to study the eect of inhomogeneities on the universe's expansion. In this thesis, new cosmologi ..."

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Since cosmological black holes modify the density and pressure of the surrounding universe, and introduce heat conduction, they produce simple models of cosmologi-cal inhomogeneities that can be used to study the eect of inhomogeneities on the universe's expansion. In this thesis, new cosmological black hole solutions are ob-tained by generalizing the expanding Kerr-Schild cosmological black holes to obtain the charged case, by performing a Kerr-Schild transformation of the Einstein-de Sitter universe (instead of a closed universe) to obtain non-expanding Kerr-Schild cosmo-logical black holes in asymptotically-
at universes, and by performing a conformal transformation on isotropic black hole spacetimes to obtain isotropic cosmological black hole spacetimes. The latter approach is found to produce cosmological black holes with energy-momentum tensors that are physical throughout spacetime, unlike previous solutions for cosmological black holes, which violate the energy conditions in some region of spacetime. In addition, it is demonstrated that radiation-dominated and matter-dominated Einstein-de Sitter universes can be directly matched across a hypersurface of constant time, and this is used to generate the rst solutions for primordial black holes that evolve from being in radiation-dominated background universes to matter-dominated background universes. Finally, the Weyl curvature, volume expansion, velocity eld, shear, and acceleration are calculated for the cos-ii mological black holes. Since the non-isotropic black holes introduce shear, according to Raychaudhuri's equation they will tend to decrease the volume expansion of the universe. Unlike several studies that have suggested the relativistic backreaction of inhomogeneities would lead to an accelerating expansion of the universe, it is con-cluded that shear should be the most likely in
uence of inhomogeneities, so they should most likely decrease the universe's expansion. iii