# Borel-Cantelli lemma

A frequently used statement on infinite sequences of random events. Let be a sequence of events from a certain probability space and let be the event consisting in the occurance of (only) a finite number out of the events , . Then, according to the Borel–Cantelli lemma, if

(*) |

then

If the events are mutually independent, then or 0, depending on whether the series converges or diverges, i.e. in this case the condition (*) is necessary and sufficient for ; this is the so-called Borel criterion for "zero or one" (cf. Zero-one law). This last criterion can be generalized to include certain classes of dependent events. The Borel–Cantelli lemma is used, for example, to prove the strong law of large numbers.

#### References

[1] | E. Borel, "Les probabilités dénombrables et leurs applications arithmetique" Rend. Circ. Mat. Palermo (2) , 27 (1909) pp. 247–271 |

[2] | F.P. Cantelli, "Sulla probabilità come limite della frequenza" Atti Accad. Naz. Lincei , 26 : 1 (1917) pp. 39–45 |

[3] | M. Loève, "Probability theory" , Princeton Univ. Press (1963) |

#### Comments

The Borel–Cantelli lemma can be used in number theory to prove the so-called "normality" of almost-all natural numbers, cf. [a1], Chapt. 8, Sect. 6.

#### References

[a1] | W. Feller, "An introduction to probability theory and its applications" , 1 , Wiley (1957) pp. Chapt.14 |

**How to Cite This Entry:**

Borel-Cantelli lemma.

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