Path Integrals and Perturbation Theory for Stochastic Processes

We review and extend the formalism introduced by Peliti, that maps a Markov process to a path-integral representation. After developing the mapping, we apply it to some illustrative examples: the simple decay process, the birth-and-death process, and the Malthus-Verhulst process. In the first two cases we show how to obtain the exact probability generating function using the path integral. We show how to implement a diagrammatic perturbation theory for processes that do not admit an exact solution. Analysis of a set of coupled Malthus-Verhulst processes on a lattice leads, in the continuum limit, to a field theory for directed percolation and allied models.Comment: 33 pages, 6 figure

Path-integral representation for a stochastic sandpile

We introduce an operator description for a stochastic sandpile model with a conserved particle density, and develop a path-integral representation for its evolution. The resulting (exact) expression for the effective action highlights certain interesting features of the model, for example, that it is nominally massless, and that the dynamics is via cooperative diffusion. Using the path-integral formalism, we construct a diagrammatic perturbation theory, yielding a series expansion for the activity density in powers of the time.Comment: 22 pages, 6 figure

Series expansion for a stochastic sandpile

Using operator algebra, we extend the series for the activity density in a one-dimensional stochastic sandpile with fixed particle density p, the first terms of which were obtained via perturbation theory [R. Dickman and R. Vidigal, J. Phys. A35, 7269 (2002)]. The expansion is in powers of the time; the coefficients are polynomials in p. We devise an algorithm for evaluating expectations of operator products and extend the series to O(t^{16}). Constructing Pade approximants to a suitably transformed series, we obtain predictions for the activity that compare well against simulations, in the supercritical regime.Comment: Extended series and improved analysi

Asymptotic behavior of the order parameter in a stochastic sandpile

We derive the first four terms in a series for the order paramater (the stationary activity density rho) in the supercritical regime of a one-dimensional stochastic sandpile; in the two-dimensional case the first three terms are reported. We reorganize the pertubation theory for the model, recently derived using a path-integral formalism [R. Dickman e R. Vidigal, J. Phys. A 35, 7269 (2002)], to obtain an expansion for stationary properties. Since the process has a strictly conserved particle density p, the Fourier mode N^{-1} psi_{k=0} -> p, when the number of sites N -> infinity, and so is not a random variable. Isolating this mode, we obtain a new effective action leading to an expansion for rho in the parameter kappa = 1/(1+4p). This requires enumeration and numerical evaluation of more than 200 000 diagrams, for which task we develop a computational algorithm. Predictions derived from this series are in good accord with simulation results. We also discuss the nature of correlation functions and one-site reduced densities in the small-kappa (large-p) limit.Comment: 18 pages, 5 figure

Processo de inovação: análise sob a ótica dos elementos organizacionais – revisão literária estruturada e implicações para pesquisas futuras

<p dir="ltr" align="justify">A história da humanidade tem demonstrado que a inovação é uma das forças motrizes e um dos meios para sustentar a evolução e criação de vantagem competitiva em um cenário de mudanças. Dada a importância da inovação no contexto empresarial e organizacional, o presente trabalho realizou um estudo bibliométrico estruturado nos artigos compreendidos entre 1990 a 2007 das duas revistas mais importantes na área de Estudos Organizacionais: <em>Administrative Science Quarterly</em> e <em>Organization Science</em>. O objetivo foi a compreensão da evolução dos estudos relacionados à inovação sob ótica dos elementos da organização. O resultado demonstrou uma tendência crescente em estudar a inovação com base em redes de conhecimento e alianças, assim como a manutenção da importância, ao longo do tempo, daqueles que abordaram a inovação e a organização como instrumento social. Outra contribuição deste trabalho é o agrupamento dos artigos estudados mediante a técnica de <em>cluster</em>, com isso foi possível traçar um mapa sobre a situação da produção acadêmica da área envolvendo a inovação relacionada aos elementos organizacionais e sugestões para pesquisas futuras.</p

DESARROLLO, EXPRESIÓN DIFERENCIAL DE PROTEÍNAS Y REDES GÉNICAS EN SCHISTOSOMA MANSONI EXPUESTOS A EUPHORBIA MILII LATEX EN ESTADO DE CERCARIA

International audienceWe tested a new approach to describe how a sublethal dose of the Euphorbia milii Des Moul. latex can affect cercariae, inducing changes in the adult Schistosoma mansoni Sambon 1907. We analyzed the effects on biology and used proteomic tools to identify the expression of different proteins. The treatment reduced the development of adults (males, 74.1%; females, 76.3%) and decreased the number of eggs released in 89.9%. The protein profile between adult males of S. mansoni from the control and exposed groups detected 1,020 spots, 26 were differently expressed and 10 were identified and validated. The proteins downregulated after exposition included actin, 14-3-3 epsilon and aldehyde dehydrogenase. The hsp70 was the only protein that was upregulated. The changes in the proteomic profile of adult males of S. mansoni may be related to a muscular fragility of the body. This work provides new insight for schistosomiasis control using a natural, water-soluble product at sublethal concentrations for mollusks. This approach is inexpensive, ecological and efficient as an indirect therapy for schistosomiasis.Hemos probado un nuevo enfoque para describir cómo una dosis subletal del látex de Euphorbia milii Des Moul. puede afectar a las cercarias, induciendo cambios en el Schistosoma mansoni Sambon 1907 adulto. Se analizaron los efectos sobre la biología y se utilizaron herramientas proteómicas para identificar la expresión de diferentes proteínas. El tratamiento redujo el desarrollo de adultos (machos 74,1%, hembras 76,3%) y disminuyó el número de huevos liberados en 89,9%. El perfil proteico entre machos adultos de S. mansoni de los grupos control y expuestos detectó 1.020 spots, 26 se expresaron de forma diferente y 10 fueron identificados y validados. Las proteínas menos abundante después de la exposición incluyeron actina, 14-3-3 epsilon y aldehído deshidrogenasa. La hsp70 fue la única proteína que se más abundante. Los cambios en el perfil proteómico de los machos adultos de S. mansoni pueden estar relacionados con una fragilidad muscular del cuerpo. Este trabajo proporciona una nueva visión para el control de la esquistosomiasis utilizando un producto natural, soluble en agua a concentraciones subletales para moluscos. Este enfoque es barato, ecológico y eficiente como terapia indirecta para la esquistosomiasi