Fonction mesurable | EPFL Graph Search

Soient E et F des espaces mesurables munis de leurs tribus respectives ℰ et ℱ. Une fonction f : E → F est dite (ℰ, ℱ)-mesurable si la par f de la tribu ℱ est incluse dans ℰ, c'est-à-dire si : \forall B \in \mathcal F\quad f^{-1}(B) \in \mathcal E. L'identité, la composée de deux fonctions mesurables, sont mesurables. Les fonctions mesurables fournissent donc à la classe des espaces mesurables une structure de catégorie. Applications à valeurs réelles Si F est l'ensemble des réels et si ℱ est sa tribu borélienne, on dira simplement que f est une fonction mesurable sur (E, ℰ). La tribu borélienne sur ℝ étant engendrée (par exemple) par l'ensemble des demi-droites de la forme ]a , +∞[, le lemme de transport assure que f est mesurable sur (E, ℰ) si et seulement si l'image réciproque par f de chacune de ces demi-droites est dans ℰ. Par exemple : toute fonction réelle d'une variable réelle qui est monotone est borélienne

Inferring the flow properties of epithelial tissues from their geometry

Valentin Camille Druelle, Marko Popovic, Matthieu Wyart

Amorphous materials exhibit complex material properties with strongly nonlinear behaviors. Below a yield stress they behave as plastic solids, while they start to yield above a critical stress sigma(c). A key quantity controlling plasticity which is, however, hard to measure is the density P(x) of weak spots, where x is the additional stress required for local plastic failure. In the thermodynamic limit P(x) similar to x(theta) is singular at x = 0 in the solid phase below the yield stress sigma(c). This singularity is related to the presence of system spanning avalanches of plastic events. Here we address the question if the density of weak spots and the flow properties of a material can be determined from the geometry of an amorphous structure alone. We show that a vertex model for cell packings in tissues exhibits the phenomenology of plastic amorphous systems. As the yield stress is approached from above, the strain rate vanishes and the avalanches size S and their duration tau diverge. We then show that in general, in materials where the energy functional depends on topology, the value x is proportional to the length L of a bond that vanishes in a plastic event. For this class of models P(x) is therefore readily measurable from geometry alone. Applying this approach to a quantification of the cell packing geometry in the developing wing epithelium of the fruit fly, we find that in this tissue P(L) exhibits a power law with exponents similar to those found numerically for a vertex model in its solid phase. This suggests that this tissue exhibits plasticity and non-linear material properties that emerge from collective cell behaviors and that these material properties govern developmental processes. Our approach based on the relation between topology and energetics suggests a new route to outstanding questions associated with the yielding transition.

IOP PUBLISHING LTD2021

Bridging Bayesian and Minimax Mean Square Error Estimation via Wasserstein Distributionally Robust Optimization

Daniel Kuhn, Peyman Mohajerin Esfahani, Viet Anh Nguyen, Soroosh Shafieezadeh Abadeh

We introduce a distributionally robust minimium mean square error estimation model with a Wasserstein ambiguity set to recover an unknown signal from a noisy observation. The proposed model can be viewed as a zero-sum game between a statistician choosing an estimator---that is, a measurable function of the observation---and a fictitious adversary choosing a prior---that is, a pair of signal and noise distributions ranging over independent Wasserstein balls---with the goal to minimize and maximize the expected squared estimation error, respectively. We show that if the Wasserstein balls are centered at normal distributions, then the zero-sum game admits a Nash equilibrium, where the players' optimal strategies are given by an affine estimator and a normal prior, respectively. We further prove that this Nash equilibrium can be computed by solving a tractable convex program. Finally, we develop a Frank-Wolfe algorithm that can solve this convex program orders of magnitude faster than state-of-the-art general purpose solvers. We show that this algorithm enjoys a linear convergence rate and that its direction-finding subproblems can be solved in quasi-closed form.

2019

Quantifying uncertainties in contact mechanics of rough surfaces using the multilevel Monte Carlo method

Sebastian Krumscheid, Fabio Nobile, Valentine Ginette Madeleine Rey

We quantify the effect of uncertainties on quantities of interest related to contact mechanics of rough surfaces. Specifically, we consider the problem of frictionless non adhesive normal contact between two semi infinite linear elastic solids subject to uncertainties. These uncertainties may for example originate from an incomplete surface description. To account for surface uncertainties, we model a rough surface as a suitable Gaussian random field whose covariance function encodes the surface’s roughness, which is experimentally measurable. Then, we introduce the multilevel Monte Carlo method which is a computationally efficient sampling method for the computation of the expectation and higher statistical moments of uncertain system outputs, such as those derived from contact simulations. In particular, we consider two different quantities of interest, namely the contact area and the number of contact clusters, and show via numerical experiments that the multilevel Monte Carlo method offers significant computational gains compared to an approximation via a classic Monte Carlo sampling.

2019

Inferring the flow properties of epithelial tissues from their geometry

Valentin Camille Druelle, Marko Popovic, Matthieu Wyart

IOP PUBLISHING LTD2021