On the stability and causality of scalar-vector theories

Various extensions of standard inflationary models have been proposed recently by adding vector fields. Because they are generally motivated by large-scale anomalies, and the possibility of statistical anisotropy of primordial fluctuations, such models require to introduce non-standard couplings between vector fields on the one hand, and either gravity or scalar fields on the other hand. In this article, we study models involving a vector field coupled to a scalar field. We derive restrictive necessary conditions for these models to be both stable (Hamiltonian bounded by below) and causal (hyperbolic equations of motion).Comment: 20 pages, references added, v2 matches published version in JCA

2D Fractional Supersymmetry for Rational Conformal Field Theory. Application for Third-Integer Spin States

A 2D- fractional supersymmetry theory is algebraically constructed. The Lagrangian is derived using an adapted superspace including, in addition to a scalar field, two fields with spins 1/3,2/3. This theory turns out to be a rational conformal field theory. The symmetry of this model goes beyond the super Virasoro algebra and connects these third-integer spin states. Besides the stress-momentum tensor, we obtain a supercurrent of spin 4/3. Cubic relations are involved in order to close the algebra; the basic algebra is no longer a Lie or a super-Lie algebra. The central charge of this model is found to be 5/3. Finally, we analyse the form that a local invariant action should take.Comment: LaTex, 20 pages. Revised in response to referees' Comment

Raman scattering through surfaces having biaxial symmetry

Magnetic Raman scattering in two-leg spin ladder materials and the relationship between the anisotropic exchange integrals are analyzed by P. J. Freitas and R. R. P. Singh in Phys. Rev. B, {\bf 62}, 14113 (2000). The angular dependence of the two-magnon scattering is shown to provide information for the magnetic anisotropy in the Sr_14Cu_24O_41 and La_6Ca_8Cu_24O_41 compounds. We point out that the experimental results of polarized Raman measurements at arbitrary angles with respect to the crystal axes have to be corrected for the light ellipticity induced inside the optically anisotropic crystals. We refer quantitatively to the case of Sr_14Cu_24O_41 and discuss potential implications for spectroscopic studies in other materials with strong anisotropy.Comment: To be published as a Comment in Phys. Rev.

Receiver Architectures for MIMO-OFDM Based on a Combined VMP-SP Algorithm

Iterative information processing, either based on heuristics or analytical frameworks, has been shown to be a very powerful tool for the design of efficient, yet feasible, wireless receiver architectures. Within this context, algorithms performing message-passing on a probabilistic graph, such as the sum-product (SP) and variational message passing (VMP) algorithms, have become increasingly popular. In this contribution, we apply a combined VMP-SP message-passing technique to the design of receivers for MIMO-ODFM systems. The message-passing equations of the combined scheme can be obtained from the equations of the stationary points of a constrained region-based free energy approximation. When applied to a MIMO-OFDM probabilistic model, we obtain a generic receiver architecture performing iterative channel weight and noise precision estimation, equalization and data decoding. We show that this generic scheme can be particularized to a variety of different receiver structures, ranging from high-performance iterative structures to low complexity receivers. This allows for a flexible design of the signal processing specially tailored for the requirements of each specific application. The numerical assessment of our solutions, based on Monte Carlo simulations, corroborates the high performance of the proposed algorithms and their superiority to heuristic approaches

Organic farming to preserve water quality?Comparison of three emblematic cases of successful management of drinking water catchment area

Protecting water resources from pollutants generated by agricultural activities is becoming more strictly regulated in Europe today, with an obligation to achieveresults. This means that towns willing to improve quality of their domestic water supply are required to regulate farmers’ practices in the water catchment areas. In this paper, we studied three cases (Munich and Augsburg in Germany, and Lons-le-Saunier in France) often listed as successful initiatives/ experiences of preservation of water quality by local authorities that have developed coordination with farmers. In this paper, we carried out a comparative analysis of the construction of city-farmer agreements, based on in-depth surveys and with a particular focus on the role of conversion to organic farming in these agreements. We highlighted several significant differences between these three case studies, with regard to the delimitation of the city’s field of action, the nature of compensation proposed to the farmers, the direct involvement of the city council in the acquisition of land in the vulnerable zone, and the importance granted to organic farming. However, in all three cases we also found similarities, such as the importance, for successful city-farmer coordination, of a facilitator as an intermediary between the two parties, as well as dialogue and contracts that span sufficiently long periods. When these conditions are met, which is the case in the two German cities, the results on the water quality are positive. From this point of view, the German water utilities’ status as “private companies owned by the city” seems to be highly conducive to the introduction of truly environment-friendly practices by farmers. In contrast, in the French case, the greater weight of regulatory constraints on the establishment of direct relations with farmers tends to prevent any fluidity in modes of action and to trigger tensions. Finally, the specific study of the role of conversion to organic farming in the solutions proposed and accepted by the farmers highlights a number of factors needed for the territorial development of this typeof farming: a strong political will that translates into high financial incentives, guaranteed local markets for organic products, and necessary technical support. These factors nevertheless remain insufficient in two of the three case studies, and only the city of Munich, starting off with a particularly favourable situation, has been able to achieve a territorial development of organic farming in tandem with the preservation of its water resources

Biharmonic pattern selection

A new model to describe fractal growth is discussed which includes effects due to long-range coupling between displacements $u$. The model is based on the biharmonic equation $\nabla^{4}u =0$ in two-dimensional isotropic defect-free media as follows from the Kuramoto-Sivashinsky equation for pattern formation -or, alternatively, from the theory of elasticity. As a difference with Laplacian and Poisson growth models, in the new model the Laplacian of $u$ is neither zero nor proportional to $u$. Its discretization allows to reproduce a transition from dense to multibranched growth at a point in which the growth velocity exhibits a minimum similarly to what occurs within Poisson growth in planar geometry. Furthermore, in circular geometry the transition point is estimated for the simplest case from the relation $r_{\ell}\approx L/e^{1/2}$ such that the trajectories become stable at the growing surfaces in a continuous limit. Hence, within the biharmonic growth model, this transition depends only on the system size $L$ and occurs approximately at a distance $60 \%$ far from a central seed particle. The influence of biharmonic patterns on the growth probability for each lattice site is also analysed.Comment: To appear in Phys. Rev. E. Copies upon request to [email protected]

Dobiński relations and ordering of boson operators

We introduce a generalization of the Dobiński relation, through which we define a family of Bell-type numbers and polynomials. Such generalized Dobiński relations are coherent state matrix elements of expressions involving boson ladder operators. This may be used in order to obtain normally ordered forms of polynomials in creation and annihilation operators, both if the latter satisfy canonical and deformed commutation relations

Resuming motor vehicle driving following orthopaedic surgery or limb trauma.

Following elective orthopaedic surgery or the treatment of a fracture, patients are temporarily unable to drive. This loss of independence may have serious social and economic consequences for the patient. It is therefore essential to know when it is safe to permit such patients to return to driving. This article, based upon a review of the current literature, proposes recommendations of the time period after which patients may safely return to driving. Practical decisions are made based upon the type of surgical intervention or fracture. Swiss legislation is equally approached so as to better define the decision