1,721,111 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Time-averaged MSD for switching diffusion
Get PDFInternational audienceWe consider a classic two-state switching diffusion model from a single-particle tracking perspective. The mean and the variance of the time-averaged mean square displacement (TAMSD) are computed exactly. When the measurement time (i.e., the trajectory duration) is comparable to or smaller than the mean residence times in each state, the ergodicity breaking parameter is shown to take arbitrarily large values, suggesting an apparent weak ergodicity breaking for this er-godic model. In this regime, individual random trajectories are not representative while the related TAMSD curves exhibit a broad spread, in agreement with experimental observations in living cells and complex fluids. Switching diffusions can thus present, in some cases, an ergodic alternative to commonly used and inherently non-ergodic continuous-time random walks that capture similar features
Depletion of Resources by a Population of Diffusing Species
No full textInternational audienceDepletion of natural and artificial resources is a fundamental problem and a potential cause of economic crises, ecological catastrophes, and death of living organisms. Understanding the depletion process is crucial for its further control and optimized replenishment of resources. In this paper, we investigate a stock depletion by a population of species that undergo an ordinary diffusion and consume resources upon each encounter with the stock. We derive the exact form of the probability density of the random depletion time, at which the stock is exhausted. The dependence of this distribution on the number of species, the initial amount of resources, and the geometric setting is analyzed. Future perspectives and related open problems are discussed
An encounter-based approach for restricted diffusion with a gradient drift
Get PDFWe develop an encounter-based approach for describing restricted diffusion
with a gradient drift towards a partially reactive boundary. For this purpose,
we introduce an extension of the Dirichlet-to-Neumann operator and use its
eigenbasis to derive a spectral decomposition for the full propagator, i.e.,
the joint probability density function for the particle position and its
boundary local time. This is the central quantity that determines various
characteristics of diffusion-influenced reactions such as conventional
propagators, survival probability, first-passage time distribution, boundary
local time distribution, and reaction rate. As an illustration, we investigate
the impact of a constant drift onto the boundary local time for restricted
diffusion on an interval. More generally, this approach accesses how external
forces may influence the statistics of encounters of a diffusing particle with
the reactive boundary
Variations on the Author
Get PDF“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
First-passage times to anisotropic partially reactive targets
Get PDFWe investigate restricted diffusion in a bounded domain towards a small
partially reactive target in three- and higher-dimensional spaces. We propose a
simple explicit approximation for the principal eigenvalue of the Laplace
operator with mixed Robin-Neumann boundary conditions. This approximation
involves the harmonic capacity and the surface area of the target, the volume
of the confining domain, the diffusion coefficient and the reactivity. The
accuracy of the approximation is checked by using a finite-elements method. The
proposed approximation determines also the mean first-reaction time, the
long-time decay of the survival probability, and the overall reaction rate on
that target. We identify the relevant length scale of the target, which
determines its trapping capacity, and investigate its relation to the target
shape. In particular, we study the effect of target anisotropy on the principal
eigenvalue by computing the harmonic capacity of prolate and oblate spheroids
in various space dimensions. Some implications of these results in chemical
physics and biophysics are briefly discussed
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
First-passage times of multiple diffusing particles with reversible target-binding kinetics
Get PDFWe investigate a class of diffusion-controlled reactions that are initiated
at the time instance when a prescribed number among particles
independently diffusing in a solvent are simultaneously bound to a target
region. In the irreversible target-binding setting, the particles that bind to
the target stay there forever, and the reaction time is the -th fastest
first-passage time to the target, whose distribution is well-known. In turn,
reversible binding, which is common for most applications, renders theoretical
analysis much more challenging and drastically changes the distribution of
reaction times. We develop a renewal-based approach to derive an approximate
solution for the probability density of the reaction time. This approximation
turns out to be remarkably accurate for a broad range of parameters. We also
analyze the dependence of the mean reaction time or, equivalently, the inverse
reaction rate, on the main parameters such as , , and binding/unbinding
constants. Some biophysical applications and further perspectives are briefly
discussed
Statistics of diffusive encounters with a small target: Three complementary approaches
Get PDFInternational audienceDiffusive search for a static target is a common problem in statistical physics with numerous applications in chemistry and biology. We look at this problem from a different perspective and investigate the statistics of encounters between the diffusing particle and the target. While an exact solution of this problem was recently derived in the form of a spectral expansion over the eigenbasis of the Dirichlet-to-Neumann operator, the latter is generally difficult to access for an arbitrary target. In this paper, we present three complementary approaches to approximate the probability density of the rescaled number of encounters with a small target in a bounded confining domain. In particular, we derive a simple fully explicit approximation, which depends only on a few geometric characteristics such as the surface area and the harmonic capacity of the target, and the volume of the confining domain. We discuss the advantages and limitations of three approaches and check their accuracy. We also deduce an explicit approximation for the distribution of the first-crossing time, at which the number of encounters exceeds a prescribed threshold. Its relations to common firstpassage time problems are discussed
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