5,902 research outputs found

### AIDS, Economic Growth and the HIPC Initiative in Honduras

AIDS, Heavily indebted poor countries, Economic growth, Foreign capital flows

### Phase behavior of hard-core lattice gases: A Fundamental Measure approach

We use an extension of fundamental measure theory to lattice hard-core fluids
to study the phase diagram of two different systems. First, two-dimensional
parallel hard squares with edge-length $\sigma=2$ in a simple square lattice.
This system is equivalent to the lattice gas with first and second neighbor
exclusion in the same lattice, and has the peculiarity that its close packing
is degenerated (the system orders in sliding columns). A comparison with other
theories is discussed. Second, a three-dimensional binary mixture of parallel
hard cubes with $\sigma_{\rm{L}}=6$ and $\sigma_{\rm{S}}=2$. Previous
simulations of this model only focused on fluid phases. Thanks to the
simplicity introduced by the discrete nature of the lattice we have been able
to map out the complete phase diagram (both uniform and nonuniform phases)
through a free minimization of the free energy functional, so the structure of
the ordered phases is obtained as a result. A zoo of entropy-driven phase
transitions is found: one-, two- and three-dimensional positional ordering, as
well as fluid-ordered phase and solid-solid demixings.Comment: 14 pages, 16 figure

### General non-existence theorem for phase transitions in one-dimensional systems with short range interactions, and physical examples of such transitions

We examine critically the issue of phase transitions in one-dimensional
systems with short range interactions. We begin by reviewing in detail the most
famous non-existence result, namely van Hove's theorem, emphasizing its
hypothesis and subsequently its limited range of applicability. To further
underscore this point, we present several examples of one-dimensional short
ranged models that exhibit true, thermodynamic phase transitions, with
increasing level of complexity and closeness to reality. Thus having made clear
the necessity for a result broader than van Hove's theorem, we set out to prove
such a general non-existence theorem, widening largely the class of models
known to be free of phase transitions. The theorem is presented from a rigorous
mathematical point of view although examples of the framework corresponding to
usual physical systems are given along the way. We close the paper with a
discussion in more physical terms of the implications of this non-existence
theorem.Comment: Short comment on possible generalization to wider classes of systems
added; accepted for publication in Journal of Statistical Physic

### Neutral networks of genotypes: Evolution behind the curtain

Our understanding of the evolutionary process has gone a long way since the
publication, 150 years ago, of "On the origin of species" by Charles R. Darwin.
The XXth Century witnessed great efforts to embrace replication, mutation, and
selection within the framework of a formal theory, able eventually to predict
the dynamics and fate of evolving populations. However, a large body of
empirical evidence collected over the last decades strongly suggests that some
of the assumptions of those classical models necessitate a deep revision. The
viability of organisms is not dependent on a unique and optimal genotype. The
discovery of huge sets of genotypes (or neutral networks) yielding the same
phenotype --in the last term the same organism--, reveals that, most likely,
very different functional solutions can be found, accessed and fixed in a
population through a low-cost exploration of the space of genomes. The
'evolution behind the curtain' may be the answer to some of the current puzzles
that evolutionary theory faces, like the fast speciation process that is
observed in the fossil record after very long stasis periods.Comment: 7 pages, 7 color figures, uses a modification of pnastwo.cls called
pnastwo-modified.cls (included

### Fundamental measure theory for mixtures of parallel hard cubes. II. Phase behavior of the one-component fluid and of the binary mixture

A previously developed fundamental measure fucntional [J. Chem. Phys.
vol.107, 6379 (1997)] is used to study the phase behavior of a system of
parallel hard cubes. The single-component fluid exhibits a continuous
transition to a solid with an anomalously large density of vacancies. The
binary mixture has a demixing transition for edge-length ratios below 0.1.
Freezing in this mixture reveals that at least the phase rich in large cubes
lies in the region where the uniform fluid is unstable, hence suggesting a
fluid-solid phase separation. A method is develop to study very asymmetric
binary mixtures by taking the limit of zero size ratio (scaling the density and
fugacity of the solvent as appropriate) in the semi-grand ensemble where the
chemical potential of the solvent is fixed. With this procedure the mixture is
exactly mapped onto a one-component fluid of parallel adhesive hard cubes. At
any density and solvent fugacity the large cubes are shown to collapse into a
close-packed solid. Nevertheless the phase diagram contains a large
metastability region with fluid and solid phases. Upon introduction of a slight
polydispersity in the large cubes the system shows the typical phase diagram of
a fluid with an isostructural solid-solid transition (with the exception of a
continuous freezing). Consequences about the phase behavior of binary mixtures
of hard core particles are then drawn.Comment: 14 pages, 6 eps figures, uses revtex, amstex, epsfig, and multicol
style file

### Phase diagrams of Zwanzig models: The effect of polydispersity

The first goal of this article is to study the validity of the Zwanzig model
for liquid crystals to predict transitions to inhomogeneous phases (like
smectic and columnar) and the way polydispersity affects these transitions. The
second goal is to analyze the extension of the Zwanzig model to a binary
mixture of rods and plates. The mixture is symmetric in that all particles have
equal volume and length-to-breadth ratio, $\kappa$. The phase diagram
containing the homogeneous phases as well as the spinodals of the transitions
to inhomogeneous phases is determined for the cases $\kappa=5$ and 15 in order
to compare with previous results obtained in the Onsager approximation. We then
study the effect of polydispersity on these phase diagrams, emphasizing the
enhancement of the stability of the biaxial nematic phase it induces.Comment: 11 pages, 12 figure

### Cross-diffusion systems for image processing: II. The nonlinear case

In this paper the use of nonlinear cross-diffu\-sion systems to model image
restoration is investigated, theoretically and numerically. In the first case,
well-posedness, scale-space properties and long time behaviour are analyzed.
From a numerical point of view, a computational study of the performance of the
models is carried out, suggesting their diversity and potentialities to treat
image filtering problems. The present paper is a continuation of a previous
work of the same authors, devoted to linear cross-diffusion models.
\keywords{Cross-diffusion \and Complex diffusion \and Image restoration

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