11,200 research outputs found
Discrete time piecewise affine models of genetic regulatory networks
We introduce simple models of genetic regulatory networks and we proceed to
the mathematical analysis of their dynamics. The models are discrete time
dynamical systems generated by piecewise affine contracting mappings whose
variables represent gene expression levels. When compared to other models of
regulatory networks, these models have an additional parameter which is
identified as quantifying interaction delays. In spite of their simplicity,
their dynamics presents a rich variety of behaviours. This phenomenology is not
limited to piecewise affine model but extends to smooth nonlinear discrete time
models of regulatory networks. In a first step, our analysis concerns general
properties of networks on arbitrary graphs (characterisation of the attractor,
symbolic dynamics, Lyapunov stability, structural stability, symmetries, etc).
In a second step, focus is made on simple circuits for which the attractor and
its changes with parameters are described. In the negative circuit of 2 genes,
a thorough study is presented which concern stable (quasi-)periodic
oscillations governed by rotations on the unit circle -- with a rotation number
depending continuously and monotonically on threshold parameters. These regular
oscillations exist in negative circuits with arbitrary number of genes where
they are most likely to be observed in genetic systems with non-negligible
delay effects.Comment: 34 page
Comment on ``Validity of Feynman's prescription of disregarding the Pauli principle in intermediate states''
In a recent paper Coutinho, Nogami and Tomio [Phys. Rev. A 59, 2624 (1999);
quant-ph/9812073] presented an example in which, they claim, Feynman's
prescription of disregarding the Pauli principle in intermediate states of
perturbation theory fails. We show that, contrary to their claim, Feynman's
prescription is consistent with the exact solution of their example.Comment: 1 pag
Dynamical complexity of discrete time regulatory networks
Genetic regulatory networks are usually modeled by systems of coupled
differential equations and by finite state models, better known as logical
networks, are also used. In this paper we consider a class of models of
regulatory networks which present both discrete and continuous aspects. Our
models consist of a network of units, whose states are quantified by a
continuous real variable. The state of each unit in the network evolves
according to a contractive transformation chosen from a finite collection of
possible transformations, according to a rule which depends on the state of the
neighboring units. As a first approximation to the complete description of the
dynamics of this networks we focus on a global characteristic, the dynamical
complexity, related to the proliferation of distinguishable temporal behaviors.
In this work we give explicit conditions under which explicit relations between
the topological structure of the regulatory network, and the growth rate of the
dynamical complexity can be established. We illustrate our results by means of
some biologically motivated examples.Comment: 28 pages, 4 figure
Si(111) strained layers on Ge(111): evidence for c(2x4) domains
The tensile strained Si(111) layers grown on top of Ge(111) substrates are
studied by combining scanning tunneling microscopy, low energy electron
diffraction and first-principles calculations. It is shown that the layers
exhibit c(2x4) domains, which are separated by domain walls along
directions. A model structure for the c(2x4) domains is proposed, which shows
low formation energy and good agreement with the experimental data. The results
of our calculations suggest that Ge atoms are likely to replace Si atoms with
dangling bonds on the surface (rest-atoms and adatoms), thus significantly
lowering the surface energy and inducing the formation of domain walls. The
experiments and calculations demonstrate that when surface strain changes from
compressive to tensile, the (111) reconstruction converts from
dimer-adatom-stacking fault-based to adatom-based structures
Estratégia e planejamento de mercado para produtor de arroz irrigado tropical.
A análise competitiva pode contribuir para o planejamento de mercado do rizicultor e possibilita a visualização de toda a complexidade dos sistemas alternativos à produção tradicional de arroz irrigado e dos requisitos para a mudança de estratégia competitiva do empreendimento rural. A aplicação dessa metodologia a cada safra permite o estabelecimento de indicadores de desempenho e comparações sobre a evolução da competitividade do agricultor
Entropy production and Kullback-Leibler divergence between stationary trajectories of discrete systems
The irreversibility of a stationary time series can be quantified using the
Kullback-Leibler divergence (KLD) between the probability to observe the series
and the probability to observe the time-reversed series. Moreover, this KLD is
a tool to estimate entropy production from stationary trajectories since it
gives a lower bound to the entropy production of the physical process
generating the series. In this paper we introduce analytical and numerical
techniques to estimate the KLD between time series generated by several
stochastic dynamics with a finite number of states. We examine the accuracy of
our estimators for a specific example, a discrete flashing ratchet, and
investigate how close is the KLD to the entropy production depending on the
number of degrees of freedom of the system that are sampled in the
trajectories.Comment: 14 pages, 7 figure
Registo da avaliação da dor em sistema de informação: um projeto em desenvolvimento
Poster apresentado nas I Jornadas do Mestrado em Enfermagem de Saúde Infantil e Pediatria, organizadas pela Escola Superior de Enfermagem do Port
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