Following the focused workshops organized by the American Institute of Mathematics, we propose a collaborative workshop, with participants of various levels of experience. We plan to organize the workshop as follows. The first two days, we will have two introductory talks in the morning; the afternoon will start with a problem session followed by work in small groups on the questions raised. The following days will be made of problem sessions, work in groups and possibly additional talks; the precise schedule will be fixed during the workshop.