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تسجيل مستخدم جديدUniversal and complete sets in martingale theory
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The Doob convergence theorem implies that the set of divergence of any martingale has measure zero. We prove that, conversely, any $G_{deltasigma}$ subset of the Cantor space with Lebesgue-measure zero can be represented as the set of divergence of some martingale. In fact, this is effective and uniform. A consequence of this is that the set of everywhere converging martingales is ${bfPi}^1_1$-complete, in a uniform way. We derive from this some universal and complete sets for the whole projective hierarchy, via a general method. We provide some other complete sets for the classes ${bfPi}^1_1$ and ${bfSigma}^1_2$ in the theory of martingales.
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