Message passing and Monte Carlo algorithms: connecting fixed points with metastable states

Mean field-like approximations (including naive mean field, Bethe and Kikuchi and more general Cluster Variational Methods) are known to stabilize ordered phases at temperatures higher than the thermodynamical transition. For example, in the Edwards-Anderson model in 2-dimensions these approximations predict a spin glass transition at finite $T$. Here we show that the spin glass solutions of the Cluster Variational Method (CVM) at plaquette level do describe well actual metastable states of the system. Moreover, we prove that these states can be used to predict non trivial statistical quantities, like the distribution of the overlap between two replicas. Our results support the idea that message passing algorithms can be helpful to accelerate Monte Carlo simulations in finite dimensional systems.Comment: 6 pages, 6 figure

New Zealand farm structure change and intensification

This report presents information about the numbers of farms in New Zealand. The different sources providing such data report very different figures and trends over time, but they all seem to indicate an overall trend of decrease in total farm numbers over the last decades, which appears to be due mainly to a decrease in the numbers of grazing and fattening farms and in the number of arable farms. The distribution of farms by size range over time shows an increase in the proportion of both the smallest and the largest ones, at the expense of the midsized categories. However, different trends can be observed depending on the farm type. The report then explores the change in livestock numbers over time. Thus, during the last 20 years, the main trends consist in an overall decrease in sheep and beef numbers, while dairy and deer numbers increased. To conclude, the report deals with the question of intensification in New Zealand farming systems. It underlines the increase in numbers of cows per hectare and production per cow in the dairy sector, and the increase in lambing rates and carcase weights in the sheep and beef sector. These changes can be linked to the increasing expenditure in agricultural research and development as well as the increasing use of services such as herd testing. Also reported is the growing use of fertilisers in New Zealand agriculture

Transverse and Polarization Mode Selection in VCSELs

We develop a mesoscopic model of semiconductor dynamics for vertical-cavity surface-emitting lasers which allows us to describe polarization and transverse mode dynamics simultaneously. Within this model, we study the selection processes and the turn-on delay for the switch-on of di erent transverse modes in gain-guided VCSELs. We consider di erent active-region diameters, excitation conditions and current shapes. Following the application of the current pulse, transverse modes become excited in a quite de nite sequence. After the turn-on, the VCSEL initially switcheson in the fundamental transverse mode, but higher-order transverse modes become excited later. In general, the results obtained are in qualitative agreement with experiments reported recently. Finally, we discuss the current shape dependence on the transverse mode selection at threshold.This work has been funded by the European Commission through the VISTA HP-TMR network. Authors also acknowledge the Comisi on Interministerial de Ciencia y Tecnolog a under project TIC98-0418-C05-02.Peer reviewe

A New Simulated Annealing Algorithm for the Multiple Sequence Alignment Problem: The approach of Polymers in a Random Media

We proposed a probabilistic algorithm to solve the Multiple Sequence Alignment problem. The algorithm is a Simulated Annealing (SA) that exploits the representation of the Multiple Alignment between $D$ sequences as a directed polymer in $D$ dimensions. Within this representation we can easily track the evolution in the configuration space of the alignment through local moves of low computational cost. At variance with other probabilistic algorithms proposed to solve this problem, our approach allows for the creation and deletion of gaps without extra computational cost. The algorithm was tested aligning proteins from the kinases family. When D=3 the results are consistent with those obtained using a complete algorithm. For $D>3$ where the complete algorithm fails, we show that our algorithm still converges to reasonable alignments. Moreover, we study the space of solutions obtained and show that depending on the number of sequences aligned the solutions are organized in different ways, suggesting a possible source of errors for progressive algorithms.Comment: 7 pages and 11 figure