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Quickest real-time detection of a Brownian coordinate drift. (English) Zbl 1499.60129

Summary: Consider the motion of a Brownian particle in two or more dimensions, whose coordinate processes are standard Brownian motions with zero drift initially, and then at some random/unobservable time, one of the coordinate processes gets a (known) nonzero drift permanently. Given that the position of the Brownian particle is being observed in real time, the problem is to detect the time at which a coordinate process gets the drift as accurately as possible. We solve this problem in the most uncertain scenario when the random/unobservable time is (i) exponentially distributed and (ii) independent from the initial motion without drift. The solution is expressed in terms of a stopping time that minimises the probability of a false early detection and the expected delay of a missed late detection. To our knowledge this is the first time that such a problem has been solved exactly in the literature.

MSC:

60G40 Stopping times; optimal stopping problems; gambling theory
60J65 Brownian motion
60H30 Applications of stochastic analysis (to PDEs, etc.)
35J15 Second-order elliptic equations
45G10 Other nonlinear integral equations
62C10 Bayesian problems; characterization of Bayes procedures

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