arXiv subjects

Benoit Perthame

Publications and source records attributed to Benoit Perthame.

40 records · Page 3Linked to original sources

arXiv2007

An inequality for the Perron and Floquet eigenvalues of monotone differential systems and age structured equations

For monotone linear differential systems with periodic coefficients, the (first) Floquet eigenvalue measures the growth rate of the system. We define an appropriate arithmetico-geometric time average of the coefficients for which we can prove that the Perron eigenvalue is smaller than the Floquet eigenvalue. We apply this method to Partial Differential Equations, and we use it for an age-structured systems of equations for the cell cycle. This opposition between Floquet and Perron eigenvalues models the loss of circadian rhythms by cancer cells.

math.SP↗

arXiv2006

Existence of solutions of the hyperbolic Keller-Segel model

We are concerned with the hyperbolic Keller-Segel model with quorum sensing, a model describing the collective cell movement due to chemical signalling with a flux limitation for high cell densities. This is a first order quasilinear equation, its flux depends on space and time via the solution to an elliptic PDE in which the right hand side is the solution to the hyperbolic equation. This model lacks strong compactness or contraction properties. Our purpose is to prove the existence of an entropy solution obtained, as usual, in passing to the limit in a sequence of solutions to the parabolic approximation. The method consists in the derivation of a kinetic formulation for the weak limit. The specific structure of the limiting kinetic equation allows for a `rigidity theorem' which identifies some property of the solution (which might be non-unique) to this kinetic equation. This is enough to deduce a posteriori the strong convergence of a subsequence.

math.AP↗

arXiv2006

On the Inverse Problem for a Size-Structured Population Model

We consider a size-structured model for cell division and address the question of determining the division (birth) rate from the measured stable size distribution of the population. We formulate such question as an inverse problem for an integro-differential equation posed on the half line. We develop firstly a regular dependency theory for the solution in terms of the coefficients and, secondly, a novel regularization technique for tackling this inverse problem which takes into account the specific nature of the equation. Our results rely also on generalized relative entropy estimates and related Poincaré inequalities.

math.AP↗

arXiv2006

Energy concentration and Sommerfeld condition for Helmholtz equation with variable index at infinity

We consider the Helmholtz equation with a variable index of refraction $n(x)$, which is not necessarily constant at infinity but can have an angular dependency like $n(x)\to n\_\infty(x/|x |)$ as $|x |\to \infty$. Under some appropriate assumptions on this convergence and on $n\_\infty$ we prove that the Sommerfeld condition at infinity still holds true under the explicit form $$ \int\_{\R^d} | \nabla u -i n\_\infty^{1/2} u \xox |^2 \f{dx}{|x |}<+\infty. $$ It is a very striking and unexpected feature that the index $n\_{\infty}$ appears in this formula and not the gradient of the phase as established by Saito in \cite {S} and broadly used numerically. This apparent contradiction is clarified by the existence of some extra estimates on the energy decay. In particular we prove that $$ \int\_{\R^d} | \nabla\_ωn\_\infty(\xox)|^2 \f{| u |^2}{|x |} dx < +\infty. $$ In fact our main contribution is to show that this can be interpreted as a concentration of the energy along the critical lines of $n\_\infty$. In other words, the Sommerfeld condition hides the main physical effect arising for a variable $n$ at infinity; energy concentration on lines rather than dispersion in all directions.

math.AP↗