125 research outputs found
Spin glasses on Bethe Lattices for large coordination number
We study spin glasses on random lattices with finite connectivity. In the
infinite connectivity limit they reduce to the Sherrington Kirkpatrick model.
In this paper we investigate the expansion around the high connectivity limit.
Within the replica symmetry breaking scheme at two steps, we compute the free
energy at the first order in the expansion in inverse powers of the average
connectivity (z), both for the fixed connectivity and for the fluctuating
connectivity random lattices. It is well known that the coefficient of the 1/z
correction for the free energy is divergent at low temperatures if computed in
the one step approximation. We find that this annoying divergence becomes much
smaller if computed in the framework of the more accurate two steps breaking.
Comparing the temperature dependance of the coefficients of this divergence in
the replica symmetric, one step and two steps replica symmetry breaking, we
conclude that this divergence is an artefact due to the use of a finite number
of steps of replica symmetry breaking. The 1/z expansion is well defined also
in the zero temperature limit.Comment: 17 pages and 6 figure
Dynamical correlations in the escape strategy of Influenza A virus
The evolutionary dynamics of human Influenza A virus presents a challenging
theoretical problem. An extremely high mutation rate allows the virus to
escape, at each epidemic season, the host immune protection elicited by
previous infections. At the same time, at each given epidemic season a single
quasi-species, that is a set of closely related strains, is observed. A
non-trivial relation between the genetic (i.e., at the sequence level) and the
antigenic (i.e., related to the host immune response) distances can shed light
into this puzzle. In this paper we introduce a model in which, in accordance
with experimental observations, a simple interaction rule based on spatial
correlations among point mutations dynamically defines an immunity space in the
space of sequences. We investigate the static and dynamic structure of this
space and we discuss how it affects the dynamics of the virus-host interaction.
Interestingly we observe a staggered time structure in the virus evolution as
in the real Influenza evolutionary dynamics.Comment: 14 pages, 5 figures; main paper for the supplementary info in
arXiv:1303.595
Maximum entropy models capture melodic styles
We introduce a Maximum Entropy model able to capture the statistics of
melodies in music. The model can be used to generate new melodies that emulate
the style of the musical corpus which was used to train it. Instead of using
the body interactions of order Markov models, traditionally used in
automatic music generation, we use a nearest neighbour model with pairwise
interactions only. In that way, we keep the number of parameters low and avoid
over-fitting problems typical of Markov models. We show that long-range musical
phrases don't need to be explicitly enforced using high-order Markov
interactions, but can instead emerge from multiple, competing, pairwise
interactions. We validate our Maximum Entropy model by contrasting how much the
generated sequences capture the style of the original corpus without
plagiarizing it. To this end we use a data-compression approach to discriminate
the levels of borrowing and innovation featured by the artificial sequences.
The results show that our modelling scheme outperforms both fixed-order and
variable-order Markov models. This shows that, despite being based only on
pairwise interactions, this Maximum Entropy scheme opens the possibility to
generate musically sensible alterations of the original phrases, providing a
way to generate innovation
Opinion dynamics with disagreement and modulated information
Opinion dynamics concerns social processes through which populations or
groups of individuals agree or disagree on specific issues. As such, modelling
opinion dynamics represents an important research area that has been
progressively acquiring relevance in many different domains. Existing
approaches have mostly represented opinions through discrete binary or
continuous variables by exploring a whole panoply of cases: e.g. independence,
noise, external effects, multiple issues. In most of these cases the crucial
ingredient is an attractive dynamics through which similar or similar enough
agents get closer. Only rarely the possibility of explicit disagreement has
been taken into account (i.e., the possibility for a repulsive interaction
among individuals' opinions), and mostly for discrete or 1-dimensional
opinions, through the introduction of additional model parameters. Here we
introduce a new model of opinion formation, which focuses on the interplay
between the possibility of explicit disagreement, modulated in a
self-consistent way by the existing opinions' overlaps between the interacting
individuals, and the effect of external information on the system. Opinions are
modelled as a vector of continuous variables related to multiple possible
choices for an issue. Information can be modulated to account for promoting
multiple possible choices. Numerical results show that extreme information
results in segregation and has a limited effect on the population, while milder
messages have better success and a cohesion effect. Additionally, the initial
condition plays an important role, with the population forming one or multiple
clusters based on the initial average similarity between individuals, with a
transition point depending on the number of opinion choices
A fast no-rejection algorithm for the Category Game
The Category Game is a multi-agent model that accounts for the emergence of
shared categorization patterns in a population of interacting individuals. In
the framework of the model, linguistic categories appear as long lived
consensus states that are constantly reshaped and re-negotiated by the
communicating individuals. It is therefore crucial to investigate the long time
behavior to gain a clear understanding of the dynamics. However, it turns out
that the evolution of the emerging category system is so slow, already for
small populations, that such an analysis has remained so far impossible. Here,
we introduce a fast no-rejection algorithm for the Category Game that
disentangles the physical simulation time from the CPU time, thus opening the
way for thorough analysis of the model. We verify that the new algorithm is
equivalent to the old one in terms of the emerging phenomenology and we
quantify the CPU performances of the two algorithms, pointing out the neat
advantages offered by the no-rejection one. This technical advance has already
opened the way to new investigations of the model, thus helping to shed light
on the fundamental issue of categorization.Comment: 17 pages, 7 figure
Modeling the emergence of contact languages
Contact languages are born out of the non-trivial interaction of two (or more) parent languages.
Nowadays, the enhanced possibility of mobility and communication allows for a
strong mixing of languages and cultures, thus raising the issue of whether there are any
pure languages or cultures that are unaffected by contact with others. As with bacteria or viruses
in biological evolution, the evolution of languages is marked by horizontal transmission;
but to date no reliable quantitative tools to investigate these phenomena have been
available. An interesting and well documented example of contact language is the emergence
of creole languages, which originated in the contacts of European colonists and
slaves during the 17th and 18th centuries in exogenous plantation colonies of especially the
Atlantic and Indian Ocean. Here, we focus on the emergence of creole languages to demonstrate
a dynamical process that mimics the process of creole formation in American and
Caribbean plantation ecologies. Inspired by the Naming Game (NG), our modeling scheme
incorporates demographic information about the colonial population in the framework of a
non-trivial interaction network including three populations: Europeans, Mulattos/Creoles,
and Bozal slaves. We show how this sole information makes it possible to discriminate territories
that produced modern creoles from those that did not, with a surprising accuracy. The
generality of our approach provides valuable insights for further studies on the emergence
of languages in contact ecologies as well as to test specific hypotheses about the peopling
and the population structures of the relevant territories. We submit that these tools could be
relevant to addressing problems related to contact phenomena in many cultural domains:
e.g., emergence of dialects, language competition and hybridization,
globalization phenomena
Innovation processes for inference
In this letter, we introduce a new approach to quantify the closeness of
symbolic sequences and test it in the framework of the authorship attribution
problem. The method, based on a recently discovered urn representation of the
Pitman-Yor process, is highly accurate compared to other state-of-the-art
methods, featuring a substantial gain in computational efficiency and
theoretical transparency. Our work establishes a clear connection between urn
models critical in interpreting innovation processes and nonparametric Bayesian
inference. It opens the way to design more efficient inference methods in the
presence of complex correlation patterns and non-stationary dynamics
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