			Home About us Contact

Parameter Regions (parameter + regions)

Distribution by Scientific Domains

Life Sciences	50%

Selected Abstracts

EVOLUTION O ANTIBIOTIC RESISTANCE BY HUMAN AND BACTERIAL NECHE CONSTRUCTION

EVOLUTION, Issue 3 2005
Maciej F. Boni
Abstract Antibiotic treatment by humans generates strong viability selection for antibiotic-resistant bacterial strains. The frequency of host antibiotic use often determines the strength of this selection, and changing patterns of antibiotic use can generate many types of behaviors in the population dynamics of resistant and sensitive bacterial populations. In this paper, we present a simple model of hosts dimorphic for their tendency to use/avoid antibiotics and bacterial pathogens dimorphic in their resistance/sensitivity to antibiotic treatment. When a constant fraction of hosts uses antibiotics, the two bacterial strain populations can coexist unless host use-frequency is above a critical value; this critical value is derived as the ratio of the fitness cost of resistance to the fitness cost of undergoing treatment. When strain frequencies can affect host behavior, the dynamics may be analyzed in the light of niche construction. We consider three models underlying changing host behavior: conformism, the avoidance of long infections, and adherence to the advice of public health officials. In the latter two, we find that the pathogen can have quite a strong effect on host behavior. In particular, if antibiotic use is discouraged when resistance levels are high, we observe a classic niche-construction phenomenon of maintaining strain polymorphism even in parameter regions where it would not be expected. [source]

Long food chains are in general chaotic

OIKOS, Issue 1 2005
Thilo Gross
The question whether chaos exists in nature is much debated. In this paper we prove that chaotic parameter regions exist generically in food chains of length greater than three. While nonchaotic dynamics is also possible, the presence of chaotic parameter regions indicates that chaotic dynamics is likely. We show that the chaotic regions survive even at high exponents of closure. Our results have been obtained using a general food chain model that describes a large class of different food chains. The existence of chaos in models of such generality can be deduced from the presence of certain bifurcations of higher codimension. [source]

Some robust design strategies for percentile estimation in binary response models

THE CANADIAN JOURNAL OF STATISTICS, Issue 4 2006
Stefanie Biedermann
Abstract For the problem of percentile estimation of a quantal response curve, the authors determine multiobjective designs which are robust with respect to misspecifications of the model assumptions. They propose a maximin approach based on efficiencies which leads to designs that are simultaneously efficient with respect to various choices of link functions and parameter regions. Furthermore, the authors deal with the problems of designing model and percentile robust experiments. They give various examples of such designs, which are calculated numerically. Quelques plans d'expérience robustes pour I'estirnation des centiles dans les modèles de reponse binaire Préoccupés par l'estimation des centiles d'une courbe de réponse quantale, les auteurs identifient des plans d'expérience multi-objectifs qui s'avèrent robustes m,me si les postulats du modèle ont été mal spécifiés. Ils proposent une approche maximin à base d'efficacités qui conduit à des plans efficaces à la fois pour divers choix de fonctions de lien et d'ensembles de valeurs pour les paramètres. Les auteurs abordent aussi la conception de modèles et de plans d'expérience robustes pour l'estimation des centiles. Ils fournissent plusieurs exemples de tels plans, obtenus numériquement. [source]

The Hubbard model extended by nearest-neighbor Coulomb and exchange interaction on a cubic cluster , rigorous and exact results

ANNALEN DER PHYSIK, Issue 6 2010
R. Schumann
The Hubbard model on a cube was revisited and extended by both nearest-neighbor Coulomb correlation W and nearest-neighbor Heisenberg exchange J. The complete eigensystem was computed exactly for all electron occupancies and all model parameters ranging from minus infinity to plus infinity. For two electrons on the cluster the eigensystem is given in analytical form. For six electrons and infinite on-site correlation U we determinded the groundstate and the groundstate energy of the pure Hubbard model analytically. For fixed electron numbers we found a multitude of ground state level crossings depending on the various model parameters. Furthermore the groundstates of the pure Hubbard model in dependence on a magnetic field h coupled to the spins are shown for the complete U-h plane. The critical magnetic field, where the zero spin groundstate breaks down is given for four and six electrons. Suprisingly we found parameter regions, where the ground state spin does not depend monotonously on J in the extended model. For the cubic cluster gas, i.e. an ensemble of clusters coupled to an electron bath, we calculated the density n (,, T, h) and the thermodynamical density of states from the grand potential. The ground states and the various spin-spin correlation functions are studied for both attractive and repulsive values of the three interaction constants. We determined the various anomalous degeneration lines, where n (,, T = 0, h = 0) shows steps higher than one, since in this parameter regions exotic phenomena as phase separation are to expect in extended models. For the cases where these lines end in triple points, i.e. groundstates of three different occupation numbers are degenerated, we give the related parameter values. Regarding the influence of the nn-exchange and the nn-Coulomb correlation onto the anomalous degeneration we find both lifting and inducing of degeneracies depending on the parameter values. [source]