Control and stabilization of waves on 1-d networks

We present some recent results on control and stabilization of waves on 1-d networks.The fine time-evolution of solutions of wave equations on networks and, consequently, their control theoretical properties, depend in a subtle manner on the topology of the network under consideration and also on the number theoretical properties of the lengths of the strings entering in it. Therefore, the overall picture is quite complex.In this paper we summarize some of the existing results on the problem of controllability that, by classical duality arguments in control theory, can be reduced to that of observability of the adjoint uncontrolled system. The problem of observability refers to that of recovering the total energy of solutions by means of measurements made on some internal or external nodes of the network. They lead, by duality, to controllability results guaranteeing that L 2-controls located on those nodes may drive sufficiently smooth solutions to equilibrium at a final time. Most of our results in this context, obtained in collaboration with R. Dáger, refer to the problem of controlling the network from one single external node. It is, to some extent, the most complex situation since, obviously, increasing the number of controllers enhances the controllability properties of the system. Our methods of proof combine sidewise energy estimates (that in the particular case under consideration can be derived by simply applying the classical d'Alembert's formula), Fourier series representations, non-harmonic Fourier analysis, and number theoretical tools.These control results belong to the class of the so-called open-loop control systems.We then discuss the problem of closed-loop control or stabilization by feedback. We present a recent result, obtained in collaboration with J. Valein, showing that the observability results previously derived, regardless of the method of proof employed, can also be recast a posteriori in the context of stabilization, so to derive explicit decay rates (as) for the energy of smooth solutions. The decay rate depends in a very sensitive manner on the topology of the network and the number theoretical properties of the lengths of the strings entering in it.In the end of the article we also present some challenging open problems

Global Carleman estimate on a network for the wave equation and application to an inverse problem.

20 pagesInternational audienceWe are interested in an inverse problem for the wave equation with potential on a starshaped network. We prove the Lipschitz stability of the inverse problem consisting in the determination of the potential on each string of the network with Neumann boundary measurements at all but one external vertices. Our main tool, proved in this article, is a global Carleman estimate for the network

On some recent advances on stabilization for hyperbolic equations

International audienceThe purpose of these Notes is to present some recent advances on stabilization for wave-like equations together with some well-known methods of stabilization. This course will give several references on the subject but do not pretend to exhaustivity. The spirit of these Notes is more that of a research monograph. We aim to give a simplified overview of some aspects of stabilization, on the point of view of energy methods, and insist on some of the methodological approaches developed recently.We will focus on nonlinear stabilization, memory-damping and indirect stabilization of coupled PDE's and present recent methods and results. Energy methods have the advantage to handle and deal with physical quantities and properties of the models under consideration. For nonlinear stabilization, our purpose is to present the optimal-weight convexity method introduced in (Alabau- Boussouira, Appl. Math. Optim. 51(1):61-105, 2005; Alabau-Boussouira, J. Differ. Equat. 248:1473-1517, 2010) which provides a whole methodology to establish easy computable energy decay rates which are optimal or quasi-optimal, and works for finite as well as infinite dimensions and allow to treat, in a unified way different PDE's, as well as different types of dampings: localized, boundary. Another important feature is that the upper estimates can be completed by lower energy estimates for several examples, and these lower estimates can be compared to the upper ones. Optimality is proved in finite dimensions and in particular for one-dimensional semi-discretized wave-like PDE's. These results are obtained through energy comparison principles (Alabau-Boussouira, J. Differ. Equat. 248: 1473-1517, 2010), which are, to our knowledge, new. This methodology can be extended to the infinite dimensional setting thanks to still energy comparison principles supplemented by interpolation techniques. The optimal-weight convexity method is presented with two approaches: a direct and an indirect one. The first approach is based on the multiplier method and requires the assumptions of the multiplier method on the zone of localization of the feedback. The second one is based on an indirect argument, namely that the solutions of the corresponding undamped systems satisfy an observability inequality, the observation zone corresponding to the damped zone for the damped system. The advantage is that, this observability inequality holds under the sharper optimal Geometric Control Condition of Bardos et al. (SIAM J. Contr. Optim. 30:1024-1065, 1992). The optimal-weight convexity method also extends to the case memory-damping, for which the damping effects are nonlocal, and leads to nonautonomous evolution equations. We will only state the results in this latter case. Indirect stabilization of coupled systems have received a lot of attention recently. This subject concerns stabilization questions for coupled PDE's with a reduced number of feedbacks. In practice, it is often not possible to control all the components of the vector state, either because of technological limitations or cost reasons. From the mathematical point of view, this means that some equations of the coupled system are not directly stabilized. This generates mathematical difficulties, which requires to introduce new tools to study such questions. In particular, it is important to understand how stabilization may be transferred from the damped equations to the undamped ones. We present several recent results of polynomial decay for smooth initial data. These results are based on energy methods, a nondifferential integral inequality introduced in (Alabau, Compt. Rendus Acad. Sci. Paris I 328:1015-1020, 1999) [see also (Alabau-Boussouira, SIAMJ. Contr. Optim. 41(2):511-541, 2002; Alabau et al. J. Evol. Equat. 2:127-150, 2002)] and coercivity properties due to the coupling operators