Diffractive Propagation on Conic Manifolds

In this survey, we review some applications and extensions of the author's results with Richard Melrose on propagation of singularities for solutions to the wave equation on manifolds with conical singularities. These results mainly concern: the local decay of energy on noncompact manifolds with diffractive trapped orbits (joint work with Dean Baskin); singularities of the wave trace created by diffractive closed geodesics (joint work with G. Austin Ford); and the distribution of scattering resonances associated to such closed geodesics (joint work with Luc Hillairet).Comment: 15 pages; contribution to Seminaire Laurent Schwartz proceeding

Resolvent estimates with mild trapping

We discuss recent progress in understanding the effects of certain trapping geometries on cut-off resolvent estimates, and thus on the qualititative behavior of linear evolution equations. We focus on trapping that is unstable, so that strong resolvent estimates hold on the real axis, and large resonance-free regions can be shown to exist beyond it.Comment: 15 pages. For Journ\'ees EDP 2012 conference proceeding

Optimal Use of Labor Market Policies: The Role of Job Search Assistance

This paper studies the role of job search assistance programs in optimal welfare-to-work programs. The analysis is based on a framework, that allows for endogenous choice of benefit types and levels, wage taxes or subsidies, and activation measures such as monitoring and job search assistance for each period of unemployment in a dynamic environment with negative duration dependence in the exit rates to employment and potential depreciation in reemployment wages. We show that the main role of job search assistance is to delay or prevent situations in which it is no longer optimal to incentivize the worker to provide positive search effort. It is used to restore or maintain some minimum exit rate to employment which increases with the cost-effectiveness of job search assistance. We also find that in line with existing policies, these programs should mainly be used at the beginning of unemployment and for short durations. However, contrary to existing schemes, they should be exclusively targeted at unemployed workers with low initial exit rates to employment. For all other workers, they should only be used if they fail to find a job within reasonable time despite high expected initial exit rates.job search, optimal unemployment insurance, welfare-to-work policies, recursive contracts

Optimal Use of Labour Market Policies

Labour market policies for the unemployed combine passive income support with active measures that aim at improving jobseekers' employment prospects. This paper extends the theoretical framework developed by Pavoni and Violante (2005a) for the optimal choice between different active and passive policies for the unemployed to a setting which allows for the use of a job search assistance programme that affects the exit rate to employment by raising search effectiveness but not productivity in the job. These programmes are one of the most widely used activation measures in OECD countries and should, therefore, be taken into account when considering the optimal design of labour market policies. The enriched model allows to answer a wide range of interesting policy questions. It is used to assess the optimality of the West German policy in the period 2000-2002 as well as the benefits from introducing tight monitoring. It is shown that sizeable budget savings could have been realised by switching to the optimal scheme, but that the net gains from monitoring are only small. In addition, some interesting results on the optimal use of job search assistance and training are derived. It is shown that existing policies already share some but not all features of the optimal scheme.Unemployment insurance, active labour market policies, recursive contracts, job search, human capital

Confounding and control

This paper deals both with the issues of confounding and of control, as the definition of a confounding factor is far from universal and there exist different methodological approaches, ex ante and ex post, for controlling for a confounding factor. In the first section the paper compares some definitions of a confounder given in the demographic and epidemiological literature with the definition of a confounder as a common cause of both treatment/exposure and response/outcome. In the second section, the paper examines confounder control from the data collection viewpoint and recalls the stratification approach for ex post control. The paper finally raises the issue of controlling for a common cause or for intervening variables, focusing in particular on latent confounders.confounding, control, structural modelling