The Road to Stueckelberg's Covariant Perturbation Theory as Illustrated by Successive Treatments of Compton Scattering

We review the history of the road to a manifestly covariant perturbative calculus within quantum electrodynamics from the early semi-classical results of the mid-twenties to the complete formalism of Stueckelberg in 1934. We chose as our case study the calculation of the cross-section of the Compton effect. We analyse Stueckelberg's paper extensively. This is our first contribution to a study of his fundamental contributions to the theoretical physics of twentieth century.Comment: This paper is a "working-physicist" version of a paper to be published in Studies in History and Philosophy of Modern Physic

Geodetic displacements and aftershocks following the 2001 M_w = 8.4 Peru earthquake: Implications for the mechanics of the earthquake cycle along subduction zones

We analyzed aftershocks and postseismic deformation recorded by the continuous GPS station AREQ following the M_w = 8.4, 23 June 2001 Peru earthquake. This station moved by 50 cm trenchward, in a N235°E direction during the coseismic phase, and continued to move in the same direction for an additional 15 cm over the next 2 years. We compare observations with the prediction of a simple one-dimensional (1-D) system of springs, sliders, and dashpot loaded by a constant force, meant to simulate stress transfer during the seismic cycle. The model incorporates a seismogenic fault zone, obeying rate-weakening friction, a zone of deep afterslip, the brittle creep fault zone (BCFZ) obeying rate-strengthening friction, and a zone of viscous flow at depth, the ductile fault zone (DFZ). This simple model captures the main features of the temporal evolution of seismicity and deformation. Our results imply that crustal strain associated with stress accumulation during the interseismic period is probably not stationary over most of the interseismic period. The BCFZ appears to control the early postseismic response (afterslip and aftershocks), although an immediate increase, by a factor of about 1.77, of ductile shear rate is required, placing constraints on the effective viscosity of the DFZ. Following a large subduction earthquake, displacement of inland sites is trenchward in the early phase of the seismic cycle and reverse to landward after a time t i for which an analytical expression is given. This study adds support to the view that the decay rate of aftershocks may be controlled by reloading due to deep afterslip. Given the ratio of preseismic to postseismic viscous creep, we deduce that frictional stresses along the subduction interface account for probably 70% of the force transmitted along the plate interface

The Changing Face of Milk Production, Milk Quality and Milking Technology in Brazil

This introductory paper gives an overview of milk production in Brazil and discusses a series of recent regulations implemented to improve milk quality with the purpose of asserting the Brazilian dairy industry as a competitor on the international market. It also points out the economic advantage of setting design guidelines for milking machines that would be best suited to Brazilian crossbred cows.Brazilian Milk Quality, Brazil Dairy Industry, Brazilian Milk Production, Brazilian Milk Prices, Agribusiness, Farm Management, Food Consumption/Nutrition/Food Safety, Industrial Organization, International Development, Political Economy,

Un défi des gestionnaires du territoire : comment composer avec les différentes définitions des risques?

Dans les pays en développement, où les territoires présentent de multiples facteurs de vulnérabilité et où l'exposition à des dangers divers et nombreux sont des phénomènes quotidiens, la mise en place de mesures visant à contrebalancer ces phénomènes devient un enjeu difficile à réaliser. Dans cette perspective, il s'agit d'étudier, à travers les pratiques des acteurs, quels sont les éléments qui permettent de définir les risques et les actions à entreprendre. Les acteurs doivent négocier ou même imposer la légitimité de leurs vulnérabilités dans un jeu de négociations, d'arbitrages et de conflits. Le cas d'étude à San Cristóbal, Guatemala, permet d'analyser les différentes solutions proposées sous l'angle des vulnérabilités en jeu. Nous montrons que la prise en compte des vulnérabilités influence la lecture de l'équation du risque des acteurs concernés et conditionne le champ des solutions possibles. Les résultats de cette recherche indiquent que le risque n'est pas univoque mais qu'il est le résultat d'un choix et des calculs faits par les acteurs. Pour cette raison, les mesures à entreprendre dépendent des compromis élaborés entre les acteurs

Representation Theory of Quantized Poincare Algebra. Tensor Operators and Their Application to One-Partical Systems

A representation theory of the quantized Poincar\'e ($\kappa$-Poincar\'e) algebra (QPA) is developed. We show that the representations of this algebra are closely connected with the representations of the non-deformed Poincar\'e algebra. A theory of tensor operators for QPA is considered in detail. Necessary and sufficient conditions are found in order for scalars to be invariants. Covariant components of the four-momenta and the Pauli-Lubanski vector are explicitly constructed.These results are used for the construction of some q-relativistic equations. The Wigner-Eckart theorem for QPA is proven.Comment: 18 page

Agrin isoforms and their role in synaptogenesis

Agrin is thought to mediate the motor neuron-induced aggregation of synaptic proteins on the surface of muscle fibers at neuromuscular junctions. Recent experiments provide direct evidence in support of this hypothesis, reveal the nature of agrin immunoreactivity at sites other than neuromuscular junctions, and have resulted in findings that are consistent with the possibility that agrin plays a role in synaptogenesis throughout the nervous system

The agrin gene codes for a family of basal lamina proteins that differ in function and distribution

We isolated two cDNAs that encode isoforms of agrin, the basal lamina protein that mediates the motor neuron-induced aggregation of acetylcholine receptors on muscle fibers at the neuromuscular junction. Both proteins are the result of alternative splicing of the product of the agrin gene, but, unlike agrin, they are inactive in standard acetylcholine receptor aggregation assays. They lack one (agrin-related protein 1) or two (agrin-related protein 2) regions in agrin that are required for its activity. Expression studies provide evidence that both proteins are present in the nervous system and muscle and that, in muscle, myofibers and Schwann cells synthesize the agrin-related proteins while the axon terminals of motor neurons are the sole source of agrin