Linking progressive and initial filtration expansions

In this paper we study progressive ?ltration expansions with random times. We show how semimartingale decompositions in the expanded ?ltration can be obtained using a natural link between progressive and initial expansions. The link is, on an intuitive level, that the two coincide after the random time. We make this idea precise and use it to establish known and new results in the case of expansion with a single random time. The methods are then extended to the multiple time case, without any restrictions on the ordering of the individual times. Finally we study the link between the expanded ?ltrations from the point of view of ?ltration shrinkage. As the main analysis progresses, we indicate how the techniques can be generalized to other types of expansions.