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In measure theory Prokhorov’s theorem relates tightness of measures to relative compactness (and hence weak convergence) in the space of probability measures. It is credited to the Soviet mathematician Yuri Vasilyevich Prokhorov, who considered probability measures on complete separable metric spaces. The term "Prokhorov’s theorem" is also applied to later generalizations to either the direct or the inverse statements. ==Statement of the theorem== Let be a separable metric space. Let denote the collection of all probability measures defined on (with its Borel σ-algebra). Theorem. # A collection of probability measures is tight if and only if the closure of is sequentially compact in the space equipped with the topology of weak convergence. # The space with the topology of weak convergence is metrizable. # Suppose that in addition, is a complete metric space (so that is a Polish space). There is a complete metric on equivalent to the topology of weak convergence; moreover, is tight if and only if the closure of in is compact. 抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)』 ■ウィキペディアで「Prokhorov's theorem」の詳細全文を読む スポンサード リンク
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