Accurate Calibration of the Velocity-dependent One-scale Model for Domain Walls

We study the asymptotic scaling properties of standard domain wall networks in several cosmological epochs. We carry out the largest field theory simulations achieved to date, with simulation boxes of size 20483, and confirm that a scale-invariant evolution of the network is indeed the attractor solution. The simulations are also used to obtain an accurate calibration for the velocity-dependent one-scale model for domain walls: we numerically determine the two free model parameters to have the values $c_w = 0.34\pm0.16$ and $k_w = 0.98\pm0.07$, which are higher precision than (but in agreement with) earlier estimates.Comment: 8 pages, version to appear in Phys. Lett. B. arXiv admin note: substantial text overlap with arXiv:1110.348

Varieties, Jobs and EU Enlargement

Two key factors that have so far allowed fast growing economies of central and eastern Europe to cope with their external constraint have been I) the presence of relatively low unit labour costs and ii)the initial undervaluation of the exchange rate. The accession to the EU will inevitably reduce both sources of competitiveness of eastern European exports: real wages are likely to catch-up western European levels and current EU members are pushing these countries to enforce labour market and social regulations that will increase labour costs; moreover, stability of the exchange rate will be a precondition for the negotiations over the accession to proceed. Small open economies can grow faster than their neighbours without running into a balance of payment crises if they succeed in increasing the number of differentiated goods produced domestically. The multiplication of the number of varieties in these countries after trade liberalisation is an unambiguous sign that consumers coming from the empty shelves of the pre-transition era have a strong taste for varieties, and hence that new varieties can create their own demand. But the increase in the number of varieties will involve a furthering of the worker reallocation process as production is still largely concentrated in homogenous good and scale-intensive industries and enterprise density is significantly lower than in western Europe. This paper will start by reviewing the changing profile and orientation of trade in transitional economies of central and eastern Europe. Next, developments in enterprise density and the performance of greenfield vs. state and privatised firms will be reviewed in an attempt to assess barriers to the entry and growth of small business. Finally, numerical simulations of a model will be developed which enables to assess the likely impact on employment, unemployment and gross worker flows of reductions in start-up costs.http://deepblue.lib.umich.edu/bitstream/2027.42/39685/3/wp301.pd

Simulated emergence of cyclic sexual-asexual reproduction

Motivated by the cyclic pattern of reproductive regimes observed in some species of green flies (``{\it aphids}''), we simulate the evolution of a population enduring harsh seasonal conditions for survival. The reproductive regime of each female is also seasonal in principle and genetically acquired, and can mutate for each newborn with some small probability. The results show a sharp transition at a critical value of the survival probability in the winter, between a reproductive regime in the fall that is predominantly sexual, for low values of this probability, or asexual, for high values.Comment: 9 pages, 4 figures, requires RevTe

The advantage of being slow: the quasi-neutral contact process

According to the competitive exclusion principle, in a finite ecosystem, extinction occurs naturally when two or more species compete for the same resources. An important question that arises is: when coexistence is not possible, which mechanisms confer an advantage to a given species against the other(s)? In general, it is expected that the species with the higher reproductive/death ratio will win the competition, but other mechanisms, such as asymmetry in interspecific competition or unequal diffusion rates, have been found to change this scenario dramatically. In this work, we examine competitive advantage in the context of quasi-neutral population models, including stochastic models with spatial structure as well as macroscopic (mean-field) descriptions. We employ a two-species contact process in which the "biological clock" of one species is a factor of $\alpha$ slower than that of the other species. Our results provide new insights into how stochasticity and competition interact to determine extinction in finite spatial systems. We find that a species with a slower biological clock has an advantage if resources are limited, winning the competition against a species with a faster clock, in relatively small systems. Periodic or stochastic environmental variations also favor the slower species, even in much larger systems.Comment: Reviewed extended versio

Varieties, Jobs and EU Enlargement

Two key factors that have so far allowed fast growing economies of central and eastern Europe to cope with their external constraint have been I) the presence of relatively low unit labour costs and ii)the initial undervaluation of the exchange rate. The accession to the EU will inevitably reduce both sources of competitiveness of eastern European exports: real wages are likely to catch-up western European levels and current EU members are pushing these countries to enforce labour market and social regulations that will increase labour costs; moreover, stability of the exchange rate will be a precondition for the negotiations over the accession to proceed. Small open economies can grow faster than their neighbours without running into a balance of payment crises if they succeed in increasing the number of differentiated goods produced domestically. The multiplication of the number of varieties in these countries after trade liberalisation is an unambiguous sign that consumers coming from the empty shelves of the pre-transition era have a strong taste for varieties, and hence that new varieties can create their own demand. But the increase in the number of varieties will involve a furthering of the worker reallocation process as production is still largely concentrated in homogenous good and scale-intensive industries and enterprise density is significantly lower than in western Europe. This paper will start by reviewing the changing profile and orientation of trade in transitional economies of central and eastern Europe. Next, developments in enterprise density and the performance of greenfield vs. state and privatised firms will be reviewed in an attempt to assess barriers to the entry and growth of small business. Finally, numerical simulations of a model will be developed which enables to assess the likely impact on employment, unemployment and gross worker flows of reductions in start-up costs.transition economies, product variety, trade specialisation

Fragmentation Experiment and Model for Falling Mercury Drops

The experiment consists of counting and measuring the size of the many fragments observed after the fall of a mercury drop on the floor. The size distribution follows a power-law for large enough fragments. We address the question of a possible crossover to a second, different power-law for small enough fragments. Two series of experiments were performed. The first uses a traditional film photographic camera, and the picture is later treated on a computer in order to count the fragments and classify them according to their sizes. The second uses a modern digital camera. The first approach has the advantage of a better resolution for small fragment sizes. The second, although with a poorer size resolution, is more reliable concerning the counting of all fragments up to its resolution limit. Both together clearly indicate the real existence of the quoted crossover. The model treats the system microscopically during the tiny time interval when the initial drop collides with the floor. The drop is modelled by a connected cluster of Ising spins pointing up (mercury) surrounded by Ising spins pointing down (air). The Ising coupling which tends to keep the spins segregated represents the surface tension. Initially the cluster carries an extra energy equally shared among all its spins, corresponding to the coherent kinetic energy due to the fall. Each spin which touches the floor loses its extra energy transformed into a thermal, incoherent energy represented by a temperature used then to follow the dynamics through Monte Carlo simulations. Whenever a small piece becomes disconnected from the big cluster, it is considered a fragment, and counted. The results also indicate the existence of the quoted crossover in the fragment-size distribution.Comment: 6 pages, 3 figure

Assessing the Number of Components in Mixture Models: a Review.

Despite the widespread application of finite mixture models, the decision of how many classes are required to adequately represent the data is, according to many authors, an important, but unsolved issue. This work aims to review, describe and organize the available approaches designed to help the selection of the adequate number of mixture components (including Monte Carlo test procedures, information criteria and classification-based criteria); we also provide some published simulation results about their relative performance, with the purpose of identifying the scenarios where each criterion is more effective (adequate).Finite mixture; number of mixture components; information criteria; simulation studies.

Examining the segment retention problem for the “Group Satellite” case

The purpose of this work is to determine how well, criteria designed to help the selection of the adequate number of market segments, perform in recovering small niche segments, in mixture regressions of normal data, with experimental data. The simulation experiment compares several segment retention criteria, including information criteria and classification-based criteria. We also address the impact of distributional misspecification on segment retention criteria success rates. This study shows that Akaike’s Information criterion with penalty factors of 3 and 4, rather than the traditional value of 2, are the best segment retention criteria to use in recovering small niche segments. Although these criteria were designed for the specific context of mixture models, they are rarely applied in the marketing literature.Information criteria; Latent Class Segmentation.