1,721,087 research outputs found
Self-avoiding walks with curvature energy on fractals
No full textSelf-avoiding walks with a curvature-dependent energy are studied with renormalization group methods on some fractal lattices. Fixed points corresponding to universal and non-universal behaviours are generally present. However initial conditions of the renormalization group recursions can prevent non-universality. When universality holds the persistence length is found to diverge much faster than in the periodic lattice as the curvature energy increases
Recurrent oligomers in proteins: an optimal scheme reconciling accurate and concise backbone representations in automated folding and design studies
No full textA novel scheme is introduced to capture the spatial correlations of
consecutive amino acids in naturally occurring proteins. This knowledge
based strategy is able to carry out optimally automated subdivisions of
protein fragments into classes of similarity. The goal is to provide the
minimal set of protein oligomers (termed "oligons" for brevity) that is
able to represent any other fragment. At variance with previous studies
in which recurrent local motifs were classified, our concern is to
provide simplified protein representations that have been optimised for
use in automated folding and/or design attempts. In such contexts, it is
paramount to limit the number of degrees of freedom per amino acid
without incurring loss of accuracy of structural representations. The
suggested method finds, by construction, the optimal compromise between
these needs. Several possible oligon lengths are considered. It is shown
that meaningful classifications cannot be done for lengths greater than
six or smaller than four. Different contexts are considered for which
oligons of length five or six are recommendable. With only a few dozen
oligons of such length, virtually any protein can be reproduced within
typical experimental uncertainties. Structural data for the oligons are
made publicly available
Statistical Mechanics of Random Paths on Disordered Lattices
No full textThe dependence of the universality class on the statistical weight of unrestricted random paths is explicitly shown both for deterministic and statistical fractals such as the incipient infinite percolation cluster. Equally weighted paths (ideal chain) and kinetically generated paths (random walks) belong, in general, to different universality classes. For deterministic fractals exact renormalization group techniques are used. Asymptotic behaviors for the end-to-end distance ranging from power to logarithmic (localization) laws are observed for the ideal chain. In all these cases, random walks in the presence of nonperfect traps are shown to be in the same universality class of the ideal chain. Logarithmic behavior is reflected insingular renormalization group recursions. For the disordered case, numerical transfer matrix techniques are exploited on percolation clusters in two and three dimensions. The two-point correlation function scales with critical exponents not obeying standard scaling relations. The distribution of the number of chains and the number of chains returning to the starting point are found to be well approximated by a log-normal distribution. The logmoment of the number of chains is found to have an essential type of singularity consistent with the log-normal distribution. A non-self-averaging behavior is argued to occur on the basis of the results
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Weak non Self-Averaging behaviour in a Random trapping Environment
No full textThe statistics of equally weighted random paths (ideal polymer) is studied in two- and three-dimensional percolating clusters. This is equivalent to diffusion in the presence of a trapping environment. The number of step walks, N, follows a logarithmic-normal distribution with a variance growing asymptotically faster than the mean, which leads to a weak non-self-averaging behavior. Critical exponents associated with the scaling of the two-point correlation function do not obey standard scaling laws
Field-induced anti-nematic ordering in assemblies of anisotropically polarizable particles
Full text linkWe have investigated the effects of dipolar interactions in a lattice system of anisotropically polarizable particles in the presence of an external field both by Monte Carlo computer simulations and through a mean-field analysis. In a specific range of low temperature, large external field and large particle density, a novel staggered nematic structure is found, in which two intercalated sublattices have different nematic order parameters. First- and second-order phase transition lines, connected at a tricritical point, enclose the anti-nematic phase in the temperature-density plane
Anomalous diffusion in presence of boundary conditions
No full textAnomalous diffusion in presence of a (fractal) boundary is investigated. Asymptotic behaviour of the survival probability and current through an absorbing boundary are exactly calculated in specific examples. They agree with recent findings related with anomalous Warburg impedance experiments. The autocorrelation function for sites near the boundary is carefully discussed ; in presence of reflecting boundary conditions it can be different from the (spatial) average of the autocorrelation at variance with the normal diffusion case. Interesting issues from a methodological point of view are discussed in the renormalization groups analysis. Scaling arguments are given relating various exponents
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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