Spectral averaging techniques for Jacobi matrices

Spectral averaging techniques for one-dimensional discrete Schroedinger operators are revisited and extended. In particular, simultaneous averaging over several parameters is discussed. Special focus is put on proving lower bounds on the density of the averaged spectral measures. These Wegner type estimates are used to analyze stability properties for the spectral types of Jacobi matrices under local perturbations

Technological Progress, Obsolescence and Depreciation

We construct a vintage capital model à la Whelan (2002) with both exogenous embodied and disembodied technical progress, and variable utilization of each vintage. The lifetime of capital goods is endogenous and it relies on the associated maintenance costs. We study the properties of the balanced growth paths. First, we show that the lifetime of capital is an increasing (resp. decreasing) function of the rate of disembodied (resp.embodied) technical progress. Second, we show that both the use-related depreciation rate and the scrapping rate incease when embodied technical progress accelerates. However, the latter drops when disembodied technical progress accelerates while the former remains unaffected. A key feature of our model is that the age-related depreciation rate does depend on the obsolescence rate in sharp contrast to the neoclassical model.Vintage capital; operation costs; embodied technical progress; age-related depreciation; obsolescence

The influence of food and vegetation on bird distribution in tropical deciduous forest and dry oak woodland in western Mexico

Bird species densities, richness and diversity were estimated on 11 7 plots in a dry forest and oak woodland in western Mexico. The counts were performed during autumn 1990 and 1991 and spring 1991 and 1992. The plant composition and stratification were measured on each plot. Arthropod densities were estimated for most trees and shrubs during the two autumns. The relationship between bird species diversity and the plant associations was inspected by means of a canonical ordination. The plant variables explaining the species richness, diversity, total density and evenness were obtained by means of multiple regressions. The two methods were complementary and the results suggest that food abundance might be related with species richness and total number of individuals. An ordination of the sampling plots, based on the bird species counts, separated the main plant associations. Nevertheless, there were no discreet sets of birds corresponding to each associations. Bird species distribution was individualistic with loose groups of species sharing different associations. The plant variables with highest correlation coefficients in the ordinations corresponded to the vegetation type and in general they were not used directly by the birds. Birds were grouped into guilds according to foraging strategies and the plant species preferences were estimated. Even though food does not seem to control the bird species distribution for non-insectivorous species, birds favour those plants offering the most appropriate food type for each guild. The influences of food on the distribution and plant choice was estimated more closely for the insectivorous guilds. In addition to a significant correlation between gleaning insectivores and lepidopteran larvae densities in the first year, insectivores had a significant preference for those plants with highest lepidopteran larvae and homopteran densities

Regions without flux surfaces of given class for\\magnetic fields in toroidal geometry

A Converse KAM method for 3D vector fields, establishing regions through which pass no invariant 2-tori transverse to a given direction field, is tested on some helical perturbations of an axisymmetric magnetic field in toroidal geometry. It finds regions corresponding to magnetic islands and chaos for the fieldline flow. Minimization of these regions is proposed as a tool to help in the design of plasma confinement devices of tokamak and stellarator type