We solve the problem of mean-variance hedging for general semimartingale modelsvia stochastic control methods. After proving that the value process of theassociated stochastic control problem has a quadratic structure, we characteriseits three coefficient processes as solutions of semimartingale...

In this paper we give a financial justification, based on non arbitrage conditions,of the (H) hypothesis in default time modelling. We also show how the (H) hypothesis isaffected by an equivalent change of probability measure.[...]