610 research outputs found
Global protein function prediction in protein-protein interaction networks
The determination of protein functions is one of the most challenging
problems of the post-genomic era. The sequencing of entire genomes and the
possibility to access gene's co-expression patterns has moved the attention
from the study of single proteins or small complexes to that of the entire
proteome. In this context, the search for reliable methods for proteins'
function assignment is of uttermost importance. Previous approaches to deduce
the unknown function of a class of proteins have exploited sequence
similarities or clustering of co-regulated genes, phylogenetic profiles,
protein-protein interactions, and protein complexes. We propose to assign
functional classes to proteins from their network of physical interactions, by
minimizing the number of interacting proteins with different categories. The
function assignment is made on a global scale and depends on the entire
connectivity pattern of the protein network. Multiple functional assignments
are made possible as a consequence of the existence of multiple equivalent
solutions. The method is applied to the yeast Saccharomices Cerevisiae
protein-protein interaction network. Robustness is tested in presence of a high
percentage of unclassified proteins and under deletion/insertion of
interactions.Comment: 5 pages, 2 figures, 2 supplementary table
Exact solution of diffusion limited aggregation in a narrow cylindrical geometry
The diffusion limited aggregation model (DLA) and the more general dielectric
breakdown model (DBM) are solved exactly in a two dimensional cylindrical
geometry with periodic boundary conditions of width 2. Our approach follows the
exact evolution of the growing interface, using the evolution matrix E, which
is a temporal transfer matrix. The eigenvector of this matrix with an
eigenvalue of one represents the system's steady state. This yields an estimate
of the fractal dimension for DLA, which is in good agreement with simulations.
The same technique is used to calculate the fractal dimension for various
values of eta in the more general DBM model. Our exact results are very close
to the approximate results found by the fixed scale transformation approach.Comment: 18 pages RevTex, 6 eps figure
Ferromagnetic ordering in graphs with arbitrary degree distribution
We present a detailed study of the phase diagram of the Ising model in random
graphs with arbitrary degree distribution. By using the replica method we
compute exactly the value of the critical temperature and the associated
critical exponents as a function of the minimum and maximum degree, and the
degree distribution characterizing the graph. As expected, there is a
ferromagnetic transition provided < \infty. However, if the fourth
moment of the degree distribution is not finite then non-trivial scaling
exponents are obtained. These results are analyzed for the particular case of
power-law distributed random graphs.Comment: 9 pages, 1 figur
Paths to Self-Organized Criticality
We present a pedagogical introduction to self-organized criticality (SOC),
unraveling its connections with nonequilibrium phase transitions. There are
several paths from a conventional critical point to SOC. They begin with an
absorbing-state phase transition (directed percolation is a familiar example),
and impose supervision or driving on the system; two commonly used methods are
extremal dynamics, and driving at a rate approaching zero. We illustrate this
in sandpiles, where SOC is a consequence of slow driving in a system exhibiting
an absorbing-state phase transition with a conserved density. Other paths to
SOC, in driven interfaces, the Bak-Sneppen model, and self-organized directed
percolation, are also examined. We review the status of experimental
realizations of SOC in light of these observations.Comment: 23 pages + 2 figure
Non conservative Abelian sandpile model with BTW toppling rule
A non conservative Abelian sandpile model with BTW toppling rule introduced
in [Tsuchiya and Katori, Phys. Rev. E {\bf 61}, 1183 (2000)] is studied. Using
a scaling analysis of the different energy scales involved in the model and
numerical simulations it is shown that this model belong to a universality
class different from that of previous models considered in the literature.Comment: RevTex, 5 pages, 6 ps figs, Minor change
Crossover component in non critical dissipative sandpile models
The effect of bulk dissipation on non critical sandpile models is studied
using both multifractal and finite size scaling analyses. We show numerically
that the local limited (LL) model exhibits a crossover from multifractal to
self-similar behavior as the control parameters and turn
towards their critical values, i.e. and . The critical exponents are not universal and exhibit a continuous
variation with . On the other hand, the finite size effects for the
local unlimited (LU), non local limited (NLL), and non local unlimited (NLU)
models are well described by the multifractal analysis for all values of
dissipation rate . The space-time avalanche structure is studied in
order to give a deeper understanding of the finite size effects and the origin
of the crossover behavior. This result is confirmed by the calculation of the
susceptibility.Comment: 13 pages, 10 figures, Published in European Physical Journal
Dynamically Driven Renormalization Group Applied to Sandpile Models
The general framework for the renormalization group analysis of
self-organized critical sandpile models is formulated. The usual real space
renormalization scheme for lattice models when applied to nonequilibrium
dynamical models must be supplemented by feedback relations coming from the
stationarity conditions. On the basis of these ideas the Dynamically Driven
Renormalization Group is applied to describe the boundary and bulk critical
behavior of sandpile models. A detailed description of the branching nature of
sandpile avalanches is given in terms of the generating functions of the
underlying branching process.Comment: 18 RevTeX pages, 5 figure
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