1,887 research outputs found
Simulating Brownian suspensions with fluctuating hydrodynamics
Fluctuating hydrodynamics has been successfully combined with several
computational methods to rapidly compute the correlated random velocities of
Brownian particles. In the overdamped limit where both particle and fluid
inertia are ignored, one must also account for a Brownian drift term in order
to successfully update the particle positions. In this paper, we present an
efficient computational method for the dynamic simulation of Brownian
suspensions with fluctuating hydrodynamics that handles both computations and
provides a similar approximation as Stokesian Dynamics for dilute and
semidilute suspensions. This advancement relies on combining the fluctuating
force-coupling method (FCM) with a new midpoint time-integration scheme we
refer to as the drifter-corrector (DC). The DC resolves the drift term for
fluctuating hydrodynamics-based methods at a minimal computational cost when
constraints are imposed on the fluid flow to obtain the stresslet corrections
to the particle hydrodynamic interactions. With the DC, this constraint need
only be imposed once per time step, reducing the simulation cost to nearly that
of a completely deterministic simulation. By performing a series of
simulations, we show that the DC with fluctuating FCM is an effective and
versatile approach as it reproduces both the equilibrium distribution and the
evolution of particulate suspensions in periodic as well as bounded domains. In
addition, we demonstrate that fluctuating FCM coupled with the DC provides an
efficient and accurate method for large-scale dynamic simulation of colloidal
dispersions and the study of processes such as colloidal gelation
Case studies of job access and reverse commute program: 2009-2010
This report presents perceptual, mobility and employment outcomes self-reported by 573 users
of 26 transportation services funded by the Job Access and Reverse Commute (JARC) program.
The respondents were predominantly low income with 42 percent reporting 2008 personal
incomes less than 20,000 or less for the
same year. Nearly half the respondents have no household vehicles. Nearly three in five
respondents reported that their travel has become reliable and convenient after using the
services. Workers using the services have benefitted from overall reductions in the cost of
commuting to work.
Close to 94 percent rated the service as being important or very important in keeping their jobs.
Respondents also self-reported that the services allowed them to access a job with better pay or
better working conditions, and to improve their skills. Both median hourly wages and median
weekly earnings are reported to have increased since using the service for those workers who
use the service to commute to work and were employed in the one-month period prior to
starting use of the service. Alternative reasons may exist for these wage changes, including
overall changes in the economic conditions of the locations where the services operate, as well
as changes in the personal conditions of the workers that are unrelated to the JARC program in
the period between starting use of the service and the time of the survey, such as graduation
from job-training or school, residential relocation and so on.
Because of the lack of a probability sample of services, the results cannot be generalized to the
entire JARC program. Detailed case studies of the 26 services yield insights into the types of
benefits that are being provided overall in these cases and the planning and programmatic
environment within which they operate
Resilience of Timed Systems
This paper addresses reliability of timed systems in the setting of resilience, that considers the behaviors of a system when unspecified timing errors such as missed deadlines occur. Given a fault model that allows transitions to fire later than allowed by their guard, a system is universally resilient (or self-resilient) if after a fault, it always returns to a timed behavior of the non-faulty system. It is existentially resilient if after a fault, there exists a way to return to a timed behavior of the non-faulty system, that is, if there exists a controller which can guide the system back to a normal behavior. We show that universal resilience of timed automata is undecidable, while existential resilience is decidable, in EXPSPACE. To obtain better complexity bounds and decidability of universal resilience, we consider untimed resilience, as well as subclasses of timed automata
Rapport sur le suivi morphosédimentaire du cordon dunaire de la plage du Vougot pour l'année 2011
Cette étude s'inscrit dans l'un des objectifs Natura 2000 défini par la commune de Guissény : la conservation et l'entretien des milieux dunaires (objectif A3) qui se traduit par une fiche action N°6 " réhabiliter les zones dunaires dégradées ". A ce titre, deux actions ont été décidées : (i) la restauration, la mise en défend et l'entretien des milieux dunaires financés par le biais des Contrats Natura 2000 (financement Etat et Europe) ; (ii) le suivi scientifique de ces opérations (pour évaluer la pertinence des actions) financé par le Conseil Régional de Bretagne entre 2004 et 2008 dans le cadre d'un Contrat Nature. Pour l'année 2009, ce suivi a été financé par la commune de Guissény. à ce titre, un suivi morphosédimentaire du cordon dunaire de la plage du Vougot a été lancé au mois de juillet 2004 ; ce travail s'est de plus accompagné de deux études qui ont été également réalisées par le laboratoire GEOMER - UMR LETG 6554 CNRS (Université de Bretagne Occidentale) ; la premiÚre portant sur la cinématique du cordon dunaire de la plage du Vougot depuis 50 ans, et la seconde sur l'évaluation du risque de submersion (Suanez, 2004 ; Suanez et Sparfel, 2005 ; Suanez et al., 2006 ; Cariolet et Suanez, 2007 ; Sparfel et Suanez, 2007 ; Suanez et al., 2007 ; Suanez et Cariolet, 2008 ; Cariolet et Suanez, 2009 ; Suanez et al., 2009 ; Suanez et al., 2010 ; Suanez et Cariolet, 2010 ; Suanez et al., 2012). Le travail mené durant l'année 2011 se situe dans la continuité de ce suivi, il a concerné (i) la poursuite du suivi topo-morphologique du cordon dunaire à partir de mesures de terrain à haute fréquence du systÚme cordon dunaire / plage intertidale ; (ii) la réalisation d'un levé du front de dune afin de poursuivre l'analyse de sa cinématique sur l'année écoulée (2010-2011) ; (III) la quantification du bilan sédimentaire de la plage orientale du Vougot depuis l'année 2009
Classification Among Hidden Markov Models
An important task in AI is one of classifying an observation as belonging to one class among several (e.g. image classification). We revisit this problem in a verification context: given k partially observable systems modeled as Hidden Markov Models (also called labeled Markov chains), and an execution of one of them, can we eventually classify which system performed this execution, just by looking at its observations? Interestingly, this problem generalizes several problems in verification and control, such as fault diagnosis and opacity. Also, classification has strong connections with different notions of distances between stochastic models.
In this paper, we study a general and practical notion of classifiers, namely limit-sure classifiers, which allow misclassification, i.e. errors in classification, as long as the probability of misclassification tends to 0 as the length of the observation grows. To study the complexity of several notions of classification, we develop techniques based on a simple but powerful notion of stationary distributions for HMMs. We prove that one cannot classify among HMMs iff there is a finite separating word from their stationary distributions. This provides a direct proof that classifiability can be checked in PTIME, as an alternative to existing proofs using separating events (i.e. sets of infinite separating words) for the total variation distance. Our approach also allows us to introduce and tackle new notions of classifiability which are applicable in a security context
On Regularity of Unary Probabilistic Automata
The quantitative verification of Probabilistic Automata (PA) is undecidable in general. Unary PA are a simpler model where the choice of action is fixed. Still, the quantitative verification problem is open and known to be as hard as Skolem\u27s problem, a problem on linear recurrence sequences, whose decidability is open for at least 40 years. In this paper, we approach this problem by studying the languages generated by unary PAs (as defined below), whose regularity would entail the decidability of quantitative verification.
Given an initial distribution, we represent the trajectory of a unary PA over time as an infinite word over a finite alphabet, where the n-th letter represents a probability range after n steps. We extend this to a language of trajectories (a set of words), one trajectory for each initial distribution from a (possibly infinite) set. We show that if the eigenvalues of the transition matrix associated with the unary PA are all distinct positive real numbers, then the language is effectively regular. Further, we show that this result is at the boundary of regularity, as non-regular languages can be generated when the restrictions are even slightly relaxed. The regular representation of the language allows us to reason about more general properties, e.g., robustness of a regular property in a neighbourhood around a given distribution
On Robustness for the Skolem, Positivity and Ultimate Positivity Problems
The Skolem problem is a long-standing open problem in linear dynamical
systems: can a linear recurrence sequence (LRS) ever reach 0 from a given
initial configuration? Similarly, the positivity problem asks whether the LRS
stays positive from an initial configuration. Deciding Skolem (or positivity)
has been open for half a century: the best known decidability results are for
LRS with special properties (e.g., low order recurrences). But these problems
are easier for ``uninitialized'' variants, where the initial configuration is
not fixed but can vary arbitrarily: checking if there is an initial
configuration from which the LRS stays positive can be decided in polynomial
time (Tiwari in 2004, Braverman in 2006).
In this paper, we consider problems that lie between the initialized and
uninitialized variant. More precisely, we ask if 0 (resp. negative numbers) can
be avoided from every initial configuration in a neighborhood of a given
initial configuration. This can be considered as a robust variant of the Skolem
(resp. positivity) problem. We show that these problems lie at the frontier of
decidability: if the neighbourhood is given as part of the input, then robust
Skolem and robust positivity are Diophantine hard, i.e., solving either would
entail major breakthrough in Diophantine approximations, as happens for
(non-robust) positivity. However, if one asks whether such a neighbourhood
exists, then the problems turn out to be decidable with PSPACE complexity.
Our techniques also allow us to tackle robustness for ultimate positivity,
which asks whether there is a bound on the number of steps after which the LRS
remains positive. There are two variants depending on whether we ask for a
``uniform'' bound on this number of steps. For the non-uniform variant, when
the neighbourhood is open, the problem turns out to be tractable, even when the
neighbourhood is given as input.Comment: Extended version of conference paper which appeared in the
proceedings of STACS'2
On Robustness for the Skolem and Positivity Problems
The Skolem problem is a long-standing open problem in linear dynamical systems: can a linear recurrence sequence (LRS) ever reach 0 from a given initial configuration? Similarly, the positivity problem asks whether the LRS stays positive from an initial configuration. Deciding Skolem (or positivity) has been open for half a century: The best known decidability results are for LRS with special properties (e.g., low order recurrences). On the other hand, these problems are much easier for "uninitialized" variants, where the initial configuration is not fixed but can vary arbitrarily: checking if there is an initial configuration from which the LRS stays positive can be decided by polynomial time algorithms (Tiwari in 2004, Braverman in 2006).
In this paper, we consider problems that lie between the initialized and uninitialized variant. More precisely, we ask if 0 (resp. negative numbers) can be avoided from every initial configuration in a neighborhood of a given initial configuration. This can be considered as a robust variant of the Skolem (resp. positivity) problem. We show that these problems lie at the frontier of decidability: if the neighborhood is given as part of the input, then robust Skolem and robust positivity are Diophantine-hard, i.e., solving either would entail major breakthrough in Diophantine approximations, as happens for (non-robust) positivity. Interestingly, this is the first Diophantine-hardness result on a variant of the Skolem problem, to the best of our knowledge. On the other hand, if one asks whether such a neighborhood exists, then the problems turn out to be decidable in their full generality, with PSPACE complexity. Our analysis is based on the set of initial configurations such that positivity holds, which leads to new insights into these difficult problems, and interesting geometrical interpretations
Optical investigation of thermoelectric topological crystalline insulator PbSnSe
PbSnSe is a novel alloy of two promising thermoelectric
materials PbSe and SnSe that exhibits a temperature dependent band inversion
below 300 K. Recent work has shown that this band inversion also coincides with
a trivial to nontrivial topological phase transition. To understand how the
properties critical to thermoelectric efficiency are affected by the band
inversion, we measured the broadband optical response of
PbSnSe as a function of temperature. We find clear optical
evidence of the band inversion at K, and use the extended Drude
model to accurately determine a dependence of the bulk carrier
lifetime, associated with electron-acoustic phonon scattering. Due to the high
bulk carrier doping level, no discriminating signatures of the topological
surface states are found, although their presence cannot be excluded from our
data.Comment: 11 pages, 6 figure
An Examination of Step Frequency and the Running Readiness Scale as Predictors of Running-Related Injury in Collegiate Cross-Country Athletes
Purpose: The purpose of this study was to determine the relationship between step frequency and the Running Readiness Scale and the occurrence of a Running-Related Injury (RRI) in a Division III cross-country team. Methods: Each athlete was screened prior to the season for their step frequency at a preferred and pre-determined pace. Additionally, each athlete performed 6 musculoskeletal tests known as the âRunning Readiness Scaleâ to assess body alignment, weight distribution, and muscular endurance. Each subject logged their training and competition schedule and injury history throughout the season using the Otterbein Run Tracker app. Results: Sixteen subjects completed data collection for the entire cross-country season. Six of the sixteen sustained a RRI (37.5%). The results of the study did not show a significant difference between the preferred and test cadences for the injured and non-injured athletes, nor was there a significant difference between the Running Readiness Scale assessments between injured and non-injured runners in this sample. Conclusions: The application of this study to a larger population of collegiate cross-country runners is needed to assess whether step frequency and the Running Readiness Scale can be used to predict injury risk in collegiate cross-country athletes
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