The purpose of this paper is two-fold. First is to extend the notions of an n-dimensional semimartingaleand its stochastic integral to a piecewise semimartingale of stochastic dimension. The propertiesof the former carry over largely intact to the latter, avoiding some of the pitfalls of...

An equivalent !-martingale measure (E!MM) for a given stochastic process Sis a probability measure R equivalent to the original measure P such that S isan R-!-martingale. Existence of an E!MM is equivalent to a classical absenceof-arbitrage property of S, and is invariant if we replace the...

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...