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Biomedical subjects

R Dickman

Publications and source records attributed to R Dickman.

At least 19 recordsLinked to original sources
PubMed2001

Critical behavior of a one-dimensional fixed-energy stochastic sandpile.

We study a one-dimensional fixed-energy version (that is, with no input or loss of particles) of Manna's stochastic sandpile model. The system has a continuous transition to an absorbing state at a critical value of the particle density, and exhibits the hallmarks of an absorbing-state phase transition, including finite-size scaling. Critical exponents are obtained from extensive simulations, which treat stationary and transient properties, and an associated interface representation. These exponents characterize the universality class of an absorbing-state phase transition with a static conserved density in one dimension; they differ from those expected at a linear-interface depinning transition in a medium with point disorder, and from those of directed percolation.

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PubMed2001

Continuously variable survival exponent for random walks with movable partial reflectors.

We study a one-dimensional lattice random walk with an absorbing boundary at the origin and a movable partial reflector. On encountering the reflector at site x, the walker is reflected (with probability r) to x-1 and the reflector is simultaneously pushed to x+1. Iteration of the transition matrix, and asymptotic analysis of the probability generating function show that the critical exponent delta governing the survival probability varies continuously between 1/2 and 1 as r varies between 0 and 1. Our study suggests a mechanism for nonuniversal kinetic critical behavior, observed in models with an infinite number of absorbing configurations.

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PubMed2001

First- and second-order phase transitions in a driven lattice gas with nearest-neighbor exclusion.

A lattice gas with infinite repulsion between particles separated by < or = 1 lattice spacing, and nearest-neighbor hopping dynamics, is subject to a drive favoring movement along one axis of the square lattice. The equilibrium (zero drive) transition to a phase with sublattice ordering, known to be continuous, shifts to lower density, and becomes discontinuous for large bias. In the ordered nonequilibrium steady state, both the particle and order-parameter densities are nonuniform, with a large fraction of the particles occupying a jammed strip oriented along the drive. The drive thus induces separation into high- and low-density regions in a system with purely repulsive interactions. Increasing the drive can provoke a transition to the ordered phase, and thereby, a sharp reduction in current.

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PubMed2000

Pernicious anemia, gastric carcinoid, and autoimmune thrombocytopenia in a young woman.

The association between gastric carcinoid tumors and pernicious anemia is well recognized. Such tumors occur in the presence of achlorhydria, chronic atrophic gastritis, hypergastrinemia, and enterochromaffin-like cell hyperplasia. In this case report, a 29-year-old woman with pernicious anemia and autoimmune thrombocytopenia who developed gastric carcinoid tumors of the gastric body is described. This is the second description of pernicious anemia associated with autoimmune thrombocytopenia. This association in a young woman together with the therapeutic options and decisions that were taken in the treatment of the patient are discussed.

Adult↗
PubMed2000

Absorbing-state phase transitions in fixed-energy sandpiles

We study sandpile models as closed systems, with the conserved energy density zeta playing the role of an external parameter. The critical energy density zeta(c) marks a nonequilibrium phase transition between active and absorbing states. Several fixed-energy sandpiles are studied in extensive simulations of stationary and transient properties, as well as the dynamics of roughening in an interface-height representation. Our primary goal is to identify the universality classes of such models, in hopes of assessing the validity of two recently proposed approaches to sandpiles: a phenomenological continuum Langevin description with absorbing states, and a mapping to driven interface dynamics in random media.

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PubMed2000

Interface scaling in the contact process

A correspondence between lattice models with absorbing states and models of pinned interfaces in random media can be established by defining local height variables h(x,t) as integrals of the activity at point x up to time t. Within this context we study the interface representation of a prototypical model with absorbing states, the contact process, in dimensions 1-3. Simulations confirm the scaling relation beta(W)=1-straight theta between the interface-width growth exponent beta(W) and the exponent straight theta governing the decay of the order parameter. A scaling property of the height distribution, which serves as the basis for this relation, is also verified. The height-height correlation function shows clear signs of anomalous scaling, in accord with Lopez' analysis [Phys. Rev. Lett. 83, 4594 (1999)], but no evidence of multiscaling.

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PubMed1999

Avalanche and spreading exponents in systems with absorbing states.

We present generic scaling laws relating spreading critical exponents and avalanche exponents (in the sense of self-organized criticality) in general systems with absorbing states. Using these scaling laws we present a collection of the state-of-the-art exponents for directed percolation, dynamical percolation, and other universality classes. This collection of results should help to elucidate the connections of self-organized criticality and systems with absorbing states. In particular, some nonuniversality in avalanche exponents is predicted for systems with many absorbing states.

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PubMed1999

Theory of the NO+CO surface-reaction model.

We derive a pair approximation (PA) for the NO+CO model with instantaneous reactions. For both the triangular and square lattices, the PA, derived here using a simpler approach, yields a phase diagram with an active state for CO-fractions y in the interval y(1)<y<y(2), with a continuous (discontinuous) phase transition to a poisoned state at y(1) (y(2)). This is in qualitative agreement with simulation for the triangular lattice, where our theory gives a rather accurate prediction for y(2). To obtain the correct phase diagram for the square lattice, i.e., no active stationary state, we reformulate the PA using sublattices. The (formerly) active regime is then replaced by a poisoned state with broken symmetry (unequal sublattice coverages), as observed recently by Kortlüke et al. [Chem. Phys. Lett. 275, 85 (1997)]. In contrast with their approach, in which the active state persists, although reduced in extent, we report here the qualitatively correct theory of the NO+CO model on the square lattice. Surface diffusion of nitrogen can lead to an active state in this case. In one dimension, the PA predicts that diffusion is required for the existence of an active state.

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PubMed1999

Pair contact process in two dimensions.

We study the stationary properties of the two-dimensional pair contact process, a nonequilibrium lattice model exhibiting a phase transition to an absorbing state with an infinite number of configurations. The critical probability and static critical exponents are determined via Monte Carlo simulations, as well as order-parameter moment ratios and the scaling of the initial density decay. The static critical properties fall in the directed percolation universality class.

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PubMed1999

Reweighting in nonequilibrium simulations.

A simple reweighting scheme is proposed for Monte Carlo simulations of interacting particle systems, permitting one to study various parameter values in a single study, and improving efficiency by an order of magnitude. Unlike earlier reweighting schemes, the present approach does not require knowledge of the stationary probability distribution, and so is applicable out of equilibrium. The method is applied to the contact process in two and three dimensions, yielding the critical parameter and spreading exponents to unprecedented precision.

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PubMed1998

Viremia, genetic heterogeneity, and immunity to hepatitis G/GB-C virus in multiply transfused patients with thalassemia.

BACKGROUND: Thalassemia patients are at high risk for posttransfusion hepatitis. Hepatitis G virus (HGV) has been suspected of being responsible for acute and chronic hepatitis. STUDY DESIGN AND METHODS: The prevalence of HGV infection, its possible association to liver disease, the genetic heterogeneity among the various HGV isolates, and immunity to HGV were studied in 36 thalassemia patients with reverse transcriptase-polymerase chain reaction assay and sequence analysis. RESULTS: HGV RNA was detected in seven patients (19.4%), only two of whom had evidence of hepatitis C virus infection as well. Sequence analysis of the NS3 gene from isolates of the five patients infected with HGV alone revealed 84.7 to 90.9 percent homology at the nucleotide level. Prolonged HGV viremia was not associated with significant liver enzyme elevation. All five patients were chronically infected with the same viral strain. E2 antibodies were detected in 57 percent of the HGV-nonviremic patients and in only 1 of 7 viremic patients. CONCLUSION: HGV is associated with persistent viremia but not with significant biochemical evidence of liver damage. There is some genetic heterogeneity among HGV isolates from thalassemia patients in Israel.

Amino Acid Sequence↗