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Borel–Cantelli Lemma and Its Generalizations

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International Encyclopedia of Statistical Science

The celebrated Borel–Cantelli Lemma is important and useful for proving the laws of large numbers in the strong form. Consider a sequence of random events {A n } on a probability space (Ω, { F}, P), and we are interested in the question of whether infinitely many random events occur or if possibly only a finite number of them occur.

The upper limit of the sequence {A n } is the random event defined by

$$\{{A}_{n}\,i.o.\} = \mathop {\lim \sup }\limits_{n \to \infty }{A}_{n} = \bigcap \limits_{n=1}^{\infty }\, \bigcup \limits_{k=n}^{\infty }{A}_{ k}\,,$$

which occurs if and only if an infinite number of events A n occur. This i.o. stands for “infinitely often.”

Below we shall use the fact that if {A n } is a sequence of random events, then

$$P\left ( \bigcup \limits_{n=1}^{\infty }{A}_{ n}\right ) \leq \ \sum \limits_{n=1}^{\infty }P({A}_{ n}).\quad ({_\ast})$$

The Borel–Cantelli Lemma

Lemma 1

If n = 1 P(​A n ​)​ < ​ , then P(lim sup n A n ​) = 0. If the random events A 1, A...

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© 2011 Springer-Verlag Berlin Heidelberg

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Chandra, T.K., Kolaneci, F. (2011). Borel–Cantelli Lemma and Its Generalizations. In: Lovric, M. (eds) International Encyclopedia of Statistical Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-04898-2_151

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