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Université de Fribourg

Quasilinear elliptic systems in divergence form with weak monotonicity and nonlinear physical data

Augsburger, Fabien ; Hungerbühler, Norbert

In: Electronic Journal of Differential Equations, 2004, vol. 144, p. 1-18

We study the quasilinear elliptic system −div σ(x, u,Du) = v(x) + f(x, u) + div g(x, u) on a bounded domain of Rⁿ with homogeneous Dirichlet boundary conditions. This system corresponds to a diffusion problem with a source v in a moving and dissolving substance, where the motion is described by g and the dissolution by f. We prove existence of a weak solution of this system under classical...

Université de Fribourg

Phylogenetic constraints and adaptation explain food-web structure

Cattin, Marie-France ; Bersier, Louis-Félix ; Banašek-Richter, Carolin ; Baltensperger, Richard ; Gabriel, Jean-Pierre

In: Nature, 2004, vol. 427, p. 835-839

Food webs are descriptions of who eats whom in an ecosystem. Although extremely complex and variable, their structure possesses basic regularities. A fascinating question is to find a simple model capturing the underlying processes behind these repeatable patterns. Until now, two models have been devised for the description of trophic interactions within a natural community. Both are essentially...

Université de Fribourg

Mathematical analysis of competition between sensory ganglion cells for neurotrophic factor in the skin

Kohli, R. ; Gabriel, Jean-Pierre ; Clarke, P.G.H.

In: Mathematical Biosciences, 2004, vol. 191, p. 207

A model is presented of competition between sensory axons for trophic molecules (e.g. a neurotrophin such as NGF), produced in a region of skin small enough to permit their free diffusion throughout it; e.g., a touch dome, or a vibrissal follicle hair sinus. The variables specified are the number of high affinity trophic factor receptors per axon terminal and the concentration of trophic factor...

Université de Fribourg

Angular Gaussian and Cauchy estimation

Auderset, Claude ; Mazza, Christian ; Ruh, Ernst A.

In: Journal of Multivariate Analysis, 2005, vol. 93, p. 180-197

This paper proposes a unified treatment of maximum likelihood estimates of angular Gaussian and multivariate Cauchy distributions in both the real and the complex case. The complex case is relevant in shape analysis. We describe in full generality the set of maxima of the corresponding log-likelihood functions with respect to an arbitrary probability measure. Our tools are the convexity of...

Université de Fribourg

Optimized point shifts and poles in the linear rational pseudospectral method for boundary value problems

Berrut, Jean-Paul ; Mittelmann, Hans D.

In: Journal of Computational Physics, 2005, vol. 204, p. 292-301

Due to their rapid – often exponential – convergence as the number N of interpolation/collocation points is increased, polynomial pseudospectral methods are very efficient in solving smooth boundary value problems. However, when the solution displays boundary layers and/or interior fronts, this fast convergence will merely occur with very large N. To address this difficulty, we present a...

Université de Fribourg

Host starvation decreases parasite load and mean host size in experimental populations

Pulkkinen, Katja ; Ebert, Dieter

In: Ecology, 2004, vol. 85, p. 823-833

While host stress in vertebrate populations has often been linked to outbreaks of epidemics, which are attributed to the immuno-compromise of the stressed hosts, no predictions have been made about the response of invertebrate host populations to stressful conditions. Experiments conducted on individual invertebrate hosts, however, suggest that starved hosts may be a poor resource for...

Université de Fribourg

An operating cost of learning in Drosophila melanogaster

Mery, Frederic ; Kawecki, Tadeusz J.

In: Animal Behaviour, 2004, vol. 68, p. 589-598

Although the fitness benefits of learning are well understood, we know little about its costs; yet both are essential to understand the evolution of animal learning. We tested the hypothesis that learning has an operating cost, such that an animal repeatedly forced to use its learning ability would show a reduction in some fitness component(s), relative to an animal of the same genotype that does...

Université de Fribourg

Evidence for epistasis: reply to Trouve et al.

Ebert, Dieter ; Salathé, P. ; Kawecki, Tadeusz J.

In: Journal of Evolutionary Biology, 2004, vol. 17, p. 1402-1404

Salathé and Ebert (2003, J. Evol. Biol. 16: 976-985) have shown that the mean logarithmic fitness of Daphnia magna clones declined faster than linearly with increasing inbreeding coefficient F. They interpreted this result as evidence for synergistic epistasis. Trouve et al. (2004, J. Evol. Biol., doi: 10.1111/j.1420-9101.2004.00755.x) suggested that hybrid vigour could be an alternative...

Université de Fribourg

Visual learning in individually assayed Drosophila larvae

Gerber, Bertram ; Scherer, Sabine ; Neuser, K. ; Michels, B. ; Hendel, T. ; Stocker, Reinhard F. ; Heisenberg, M.

In: Journal of Experimental Biology, 2004, vol. 207, p. 179-188

An understanding of associative learning is facilitated if it can be analyzed in a simple animal like the fruit fly Drosophila. Here, we introduce the first visual associative learning paradigm for larval Drosophila; this is remarkable as larvae have an order of magnitude fewer neurons than adult flies. Larvae were subjected to either of two reciprocal training regimes: Light+/Dark- or...

Université de Fribourg

Non-canonical eukaryotic glutaminyl- and glutamyl-tRNA synthetases form mitochondrial aminoacyl-tRNA in Trypanosoma brucei

Rinehart, Jesse ; Horn, Elke K. ; Wei, David ; Söll, Dieter ; Schneider, André

In: Journal of Biological Chemistry, 2004, vol. 279, p. 1161-1166

Glutaminyl-tRNA synthetase is thought to be absent from organelles. Instead, Gln-tRNA is formed via the transamidation pathway, the other route to this essential compound in protein biosynthesis. However, it was previously shown that glutaminyl-tRNA synthetase activity is present in Leishmania mitochondria. This work identifies genes encoding glutaminyl- and glutamyl-tRNA synthetase in the...