infonet-economy

In this article, we define the notion of a filtration and the related notion of the usual hypotheses. We then explain the problem of enlargements of filtrations: how are (semi)martingales affected under a change of filtrations? We state the main theorems in the classical frameworks of initial and progressive enlargements of filtrations. In the case of initial enlargements of filtrations, we state the well known Jacod's condition and in the framework of progressive enlargements of filtrations, we give the decomposition of a local martingale in the larger filtration. We finally specialize to the case of immersed filtrations, which is very widely used in credit risk modeling, and study the effect of a combination of changes of filtration and probability measure in this situation.