Abstract. Let X be a process defined on an optional random set. The paper develops two different conditions on X guaranteeing that it is the restriction of a uniformly integrable martingale. In each case, it is supposed that X is the restriction to Λ of some special semimartingale Z with canonical decomposition Z = M + A. The first condition, which is both necessary and sufficient, is an absolute continuity condition on A. Under additional hypotheses, the existence of a martingale extension can be characterized by a strong martingale property of X on Λ. Uniqueness of the extension is also considered.
IntroductionLet Λ be an optional random set and let X t (ω) be defined for (t, ω) ∈ Λ. We consider the following and some of its extensions.
(0.1) Problem. Find necessary and sufficient conditions on X guaranteeing that it is the restriction to Λ of a globally defined, right continuous uniformly integrable martingale.For an example where this formulation may be natural, consider a process (Y t ) t≥0 with values in a manifold. Given a coordinate patch V , let Λ := {(t, ω) : Y t (ω) ∈ V } and let X t (ω) denote a real component of Y t (ω) for (t, ω) ∈ Λ. A second natural example is provided by X = f •W , where W is a Markov process in a state space E and f is a function defined on a subset S of E, Λ denoting in this case {(t, ω) :The solution is obvious if there is an increasing sequence of stopping times T n which are complete sections of Λ (i.e., T n ⊂ Λ and P{T n < ∞} = 1) such that Λ ⊂ ∪ n 0, T n . A number of other cases may now be found in the literature. In sections 2 and 3, we give a complete solution for the discrete parameter problem under mild conditions on X. The continuous parameter case, treated in sections 4 and 5, involves considerable additional complication. Roughly speaking, in the discrete parameter case the condition is that X have a "strong martingale property" on Λ (defined in (3.4)), but in the continuous parameter context, an example is given in section 5 to show that this condition is not sufficient. Theorem (4.1), one of the main results of the paper, assumes X is the restriction of a special semimartingale Z = M + A (with A predictable and of integrable 1991 Mathematics Subject Classification. 60J30.