1,589 research outputs found
Species lifetime distribution for simple models of ecologies
Interpretation of empirical results based on a taxa's lifetime distribution
shows apparently conflicting results. Species' lifetime is reported to be
exponentially distributed, whereas higher order taxa, such as families or
genera, follow a broader distribution, compatible with power law decay. We show
that both these evidences are consistent with a simple evolutionary model that
does not require specific assumptions on species interaction. The model
provides a zero-order description of the dynamics of ecological communities and
its species lifetime distribution can be computed exactly. Different behaviors
are found: an initial power law, emerging from a random walk type of
dynamics, which crosses over to a steeper branching process-like
regime and finally is cutoff by an exponential decay which becomes weaker and
weaker as the total population increases. Sampling effects can also be taken
into account and shown to be relevant: if species in the fossil record were
sampled according to the Fisher log-series distribution, lifetime should be
distributed according to a power law. Such variability of behaviors in
a simple model, combined with the scarcity of data available, cast serious
doubts on the possibility to validate theories of evolution on the basis of
species lifetime data.Comment: 19 pages, 2 figure
Reply to Comment on ``Thermal Model for Adaptive Competition in a Market''
We reply to the Comment of Challet et al. [cond-mat/0004308] on our paper
[Phys. Rev. Lett. 83, 4429 (1999)]. We show that the claim of the Comment that
the effects of the temperature in the Thermal Minority Game ``can be eliminated
by time rescaling'' and consequently the behaviour is ``independent of T'' has
no general validity.Comment: 1 page, 1 figur
Laplacian Fractal Growth in Media with Quenched Disorder
We analyze the combined effect of a Laplacian field and quenched disorder for
the generation of fractal structures with a study, both numerical and
theoretical, of the quenched dielectric breakdown model (QDBM). The growth
dynamics is shown to evolve from the avalanches of invasion percolation (IP) to
the smooth growth of Laplacian fractals, i. e. diffusion limited aggregation
(DLA) and the dielectric breakdown model (DBM). The fractal dimension is
strongly reduced with respect to both DBM and IP, due to the combined effect of
memory and field screening. This implies a specific relation between the
fractal dimension of the breakdown structures (dielectric or mechanical) and
the microscopic properties of disordered materials.Comment: 11 pages Latex (revtex), 3 postscript figures included. Submitted to
PR
The iso-Nazarov reaction
The construction of five-membered rings is essential in organic chemistry. In this context, pentannulation reactions that provide a straightforward access to cyclopentenones are of particular interest, as these structures are not only embedded in important molecules such as some prostaglandins, but also serve as versatile building blocks in organic synthesis. This review documents the acid-promoted cycloisomerization of conjugated dienals and linearly-conjugated dienones for the construction of cyclopentenones, a transformation that has been largely eclipsed by the well-known Nazarov reaction, i.e. the acid-promoted cycloisomerization of cross-conjugated ketones.Fil: Riveira, MartĂn Jorge. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Rosario. Instituto de QuĂmica Rosario. Universidad Nacional de Rosario. Facultad de Ciencias BioquĂmicas y FarmacĂ©uticas. Instituto de QuĂmica Rosario; ArgentinaFil: Marsili, LucĂa A.. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Rosario. Instituto de QuĂmica Rosario. Universidad Nacional de Rosario. Facultad de Ciencias BioquĂmicas y FarmacĂ©uticas. Instituto de QuĂmica Rosario; ArgentinaFil: Mischne, Mirta Paulina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Rosario. Instituto de QuĂmica Rosario. Universidad Nacional de Rosario. Facultad de Ciencias BioquĂmicas y FarmacĂ©uticas. Instituto de QuĂmica Rosario; Argentin
From Minority Games to real markets
We address the question of market efficiency using the Minority Game (MG)
model. First we show that removing unrealistic features of the MG leads to
models which reproduce a scaling behavior close to what is observed in real
markets. In particular we find that i) fat tails and clustered volatility arise
at the phase transition point and that ii) the crossover to random walk
behavior of prices is a finite size effect. This, on one hand, suggests that
markets operate close to criticality, where the market is marginally efficient.
On the other it allows one to measure the distance from criticality of real
market, using cross-over times. The artificial market described by the MG is
then studied as an ecosystem with different_species_ of traders. This clarifies
the nature of the interaction and the particular role played by the various
populations.Comment: 9 pages, 7 figures, to appear in Quantitative Financ
Theory of Self-organized Criticality for Problems with Extremal Dynamics
We introduce a general theoretical scheme for a class of phenomena
characterized by an extremal dynamics and quenched disorder. The approach is
based on a transformation of the quenched dynamics into a stochastic one with
cognitive memory and on other concepts which permit a mathematical
characterization of the self-organized nature of the avalanche type dynamics.
In addition it is possible to compute the relevant critical exponents directly
from the microscopic model. A specific application to Invasion Percolation is
presented but the approach can be easily extended to various other problems.Comment: 11 pages Latex (revtex), 3 postscript figures included. Submitted to
Europhys. Let
Generalized Dielectric Breakdown Model
We propose a generalized version of the Dielectric Breakdown Model (DBM) for
generic breakdown processes. It interpolates between the standard DBM and its
analog with quenched disorder, as a temperature like parameter is varied. The
physics of other well known fractal growth phenomena as Invasion Percolation
and the Eden model are also recovered for some particular parameter values. The
competition between different growing mechanisms leads to new non-trivial
effects and allows us to better describe real growth phenomena.
Detailed numerical and theoretical analysis are performed to study the
interplay between the elementary mechanisms. In particular, we observe a
continuously changing fractal dimension as temperature is varied, and report an
evidence of a novel phase transition at zero temperature in absence of an
external driving field; the temperature acts as a relevant parameter for the
``self-organized'' invasion percolation fixed point. This permits us to obtain
new insight into the connections between self-organization and standard phase
transitions.Comment: Submitted to PR
On the rise and fall of networked societies
We review recent results on the dynamics of social networks which suggest
that the interplay between the network formation process and volatility may
lead to the occurrence of discontinuous phase transitions and phase coexistence
in a large class of models. We then investigate the effects of negative links
-- links inhibiting local growth of the network -- and of a geographical
distribution of the agents in such models. We show, by extensive numerical
simulations, that both effects enhance this phenomenology, i.e. it increases
the size of the coexistence region.Comment: 6 pages, 4 figures, Proceedings of Granada Workshop 200
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