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Kolmogorov's zero–one law
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Kolmogorov's zero–one law : ウィキペディア英語版
Kolmogorov's zero–one law

In probability theory, Kolmogorov's zero–one law, named in honor of Andrey Nikolaevich Kolmogorov, specifies that a certain type of event, called a ''tail event'', will either almost surely happen or almost surely not happen; that is, the probability of such an event occurring is zero or one.
Tail events are defined in terms of infinite sequences of random variables. Suppose
:X_1,X_2,X_3,\dots\,
is an infinite sequence of independent random variables (not necessarily identically distributed). Let \mathcal be the σ-algebra generated by the X_i. Then, a tail event F \in \mathcal is an event which is probabilistically independent of each finite subset of these random variables. (Note: F belonging to \mathcal implies that membership in F is uniquely determined by the values of the X_i but the latter condition is strictly weaker and does not suffice to prove the zero-one law.) For example, the event that the sequence converges, and the event that its sum converges are both tail events. In an infinite sequence of coin-tosses, a sequence of 100 consecutive heads occurring infinitely many times is a tail event.
In many situations, it can be easy to apply Kolmogorov's zero–one law to show that some event has probability 0 or 1, but surprisingly hard to determine ''which'' of these two extreme values is the correct one.
== Formulation ==
A more general statement of Kolmogorov's zero–one law holds for sequences of independent σ-algebras. Let (Ω,''F'',''P'') be a probability space and let ''F''''n'' be a sequence of mutually independent σ-algebras contained in ''F''. Let
:G_n=\sigma\bigg(\bigcup_^\infty F_k\bigg)
be the smallest σ-algebra containing ''F''''n'', ''F''''n''+1, …. Then Kolmogorov's zero–one law asserts that for any event
:F\in \bigcap_^\infty G_n
one has either ''P''(''F'') = 0 or 1.
The statement of the law in terms of random variables is obtained from the latter by taking each ''F''''n'' to be the σ-algebra generated by the random variable ''X''''n''. A tail event is then by definition an event which is measurable with respect to the σ-algebra generated by all ''X''''n'', but which is independent of any finite number of ''X''''n''. That is, a tail event is precisely an element of the intersection \textstyle.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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