Bertrand Maury

Concurrent sexual partnerships and HIV prevalence in five urban communities of sub-Saharan Africa.

ObjectiveTo estimate parameters of concurrent sexual partnerships in five urban populations in sub-Saharan Africa and to assess their association with levels of HIV infection and other sexually transmitted infections (STI). MethodsData were obtained from a multicentre study of factors which determine the differences in rate of spread of HIV in five African cities. Consenting participants were interviewed on sexual behaviour and at four of the five sites also provided a blood and a urine sample for testing for HIV and other STI. Data on sexual behaviour included the number of partnerships in the 12 months preceding the interview as well as the dates of the start and end of each partnership. Summary indices of concurrent sexual partnerships – some of which were taken from the literature, while others were newly developed – were computed for each city and compared to HIV and STI prevalence rates. ResultsA total of 1819 adults aged 15–49 years were interviewed in Dakar (Senegal), 2116 in Cotonou (Benin), 2089 in Yaoundé (Cameroon), 1889 in Kisumu (Kenya) and 1730 in Ndola (Zambia). Prevalence rates of HIV infection were 3.4% for Cotonou, 5.9% for Yaoundé, 25.9% for Kisumu and 28.4% for Ndola, and around 1% for Dakar. The estimated fraction of sexual partnerships that were concurrent at the time of interview (index k) was relatively high in Yaoundé (0.98), intermediate in Kisumu (0.44) and Cotonou (0.33) and low in Ndola (0.26) and in Dakar (0.18). An individual indicator of concurrency (iic) was developed which depends neither on the number of partners nor on the length of the partnerships and estimates the individual propensity to keep (positive values) or to dissolve (negative values) on-going partnership before engaging in another one. This measure iic did not discriminate between cities with high HIV infection levels and cities with low HIV infection levels. In addition, iic did not differ significantly between HIV-infected and uninfected people in the four cities where data on HIV status were collected. ConclusionWe could not find evidence that concurrent sexual partnerships were a major determinant of the rate of spread of HIV in five cities in sub-Saharan Africa. HIV epidemics are the result of many factors, behavioural as well as biological, of which concurrent sexual partnerships are only one.

A MACROSCOPIC CROWD MOTION MODEL OF GRADIENT FLOW TYPE

A simple model to handle the flow of people in emergency evacuation situations is considered: at every point x, the velocity U(x) that individuals at x would like to realize is given. Yet, the incompressibility constraint prevents this velocity field to be realized and the actual velocity is the projection of the desired one onto the set of admissible velocities. Instead of looking at a microscopic setting (where individuals are represented by rigid discs), here the macroscopic approach is investigated, where the unknwon is the evolution of the density . If a gradient structure is given, say U is the opposite of the gradient of D where D is, for instance, the distance to the exit door, the problem is presented as a Gradient Flow in the Wasserstein space of probability measures. The functional which gives the Gradient Flow is neither finitely valued (since it takes into account the constraints on the density), nor geodesically convex, which requires for an ad-hoc study of the convergence of a discrete scheme.

Mind wandering and driving: responsibility case-control study

Objective To assess the association between mind wandering (thinking unrelated to the task at hand) and the risk of being responsible for a motor vehicle crash. Design Responsibility case-control study. Setting Adult emergency department of a university hospital in France, April 2010 to August 2011. Participants 955 drivers injured in a motor vehicle crash. Main outcome measures Responsibility for the crash, mind wandering, external distraction, negative affect, alcohol use, psychotropic drug use, and sleep deprivation. Potential confounders were sociodemographic and crash characteristics. Results Intense mind wandering (highly disrupting/distracting content) was associated with responsibility for a traffic crash (17% (78 of 453 crashes in which the driver was thought to be responsible) v 9% (43 of 502 crashes in which the driver was not thought to be responsible); adjusted odds ratio 2.12, 95% confidence interval 1.37 to 3.28). Conclusions Mind wandering while driving, by decoupling attention from visual and auditory perceptions, can jeopardise the ability of the driver to incorporate information from the environment, thereby threatening safety on the roads.

A time-stepping scheme for inelastic collisions

We propose here a numerical scheme to compute the motion of rigid bodies with a non-elastic impact law. The method is based on a global computation of the reaction forces between bodies. Those forces, whose direction is known since we neglect friction effects, are identified at the discrete level with a scalar which plays the role of a Kuhn-Tucker multiplier associated to a first-order approximation of the non-overlapping constraint, expressed in terms of velocities. Since our original motivation is the handling of the non-overlapping constraint in fluid-particle direct simulations, we paid a special attention to stability and robustness. The scheme is proved to be stable and robust. As regards its asymptotic behaviour, a convergence result is established in the case of a single contact. Some numerical tests are presented to illustrate the properties of the algorithm. Firstly, we investigate its asymptotic behaviour in a situation of non-uniqueness, for a single particle. The two other sets of results show the good behaviour of the scheme for large time steps.

Handling congestion in crowd motion modeling

We address here the issue of congestion in the modeling of crowd motion, in the non-smooth framework: contacts between people are not anticipated and avoided, they actually occur, and they are explicitly taken into account in the model. We limit our approach to very basic principles in terms of behavior, to focus on the particular problems raised by the non-smooth character of the models. We consider that individuals tend to move according to a desired, or spontaneous, velocity. We account for congestion by assuming that the evolution realizes at each time an instantaneous balance between individual tendencies and global constraints (overlapping is forbidden): the actual velocity is defined as the closest to the desired velocity among all admissible ones, in a least square sense. We develop those principles in the microscopic and macroscopic settings, and we present how the framework of Wasserstein distance between measures allows to recover the sweeping process nature of the problem on the macroscopic level, which makes it possible to obtain existence results in spite of the non-smooth character of the evolution process. Micro and macro approaches are compared, and we investigate the similarities together with deep differences of those two levels of description.

Fluid-particle flow: a symmetric formulation

Abstract In this Note, we present a numerical method to simulate the motion of solid particles in a moving viscous fluid. The fluid is supposed to be Newtonian and incompressible. The Arbitrary Lagrangian Eulerian formulation of the Navier-Stokes equations is discretized at the first order in time, as are the equations for the solid bodies. The advection term is taken into account by a method of characteristics. The variational formulation of the coupled problem is then established, and the boundary integrals expressing the hydrodynamical forces are eliminated. By introduction of an appropriate Finite Element approximation, a symmetric linear system is obtained. This system is solved by an inexact Uzawa algorithm, preconditionned by a Laplace operator with Neumann boundary conditions on the pressure. Numerical results are presented, for 2 and 100 particles: The Reynolds number in both cases is of the order of 100.

A Fat Boundary Method for the Poisson Problem in a Domain with Holes

AbstractWe consider the Poisson equation with Dirichlet boundary conditions, in a domain Ω\

Numerical Analysis of a Finite Element/Volume Penalty Method

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