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Proofs of convergence of random variables : ウィキペディア英語版
Proofs of convergence of random variables

This article is supplemental for “Convergence of random variables” and provides proofs for selected results.
Several results will be established using the portmanteau lemma: A sequence converges in distribution to ''X'' if and only if any of the following conditions are met:

  1. E() → E() for all bounded, continuous functions ''f'';
  2. E() → E() for all bounded, Lipschitz functions ''f'';
  3. limsup ≤ Pr(''X'' ∈ ''C'') for all closed sets ''C'';

== Convergence almost surely implies convergence in probability==
: X_n\ \xrightarrow\ X \quad\Rightarrow\quad X_n\ \xrightarrow\ X
Proof: If converges to ''X'' almost surely, it means that the set of points has measure zero; denote this set ''O''. Now fix ε > 0 and consider a sequence of sets
: A_n = \bigcup_ \left \
This sequence of sets is decreasing: ''A''''n'' ⊇ ''A''''n''+1 ⊇ ..., and it decreases towards the set
:A_ = \bigcap_ A_n.
For this decreasing sequence of events, their probabilities are also a decreasing sequence, and it decreases towards the Pr(''A''); we shall show now that this number is equal to zero. Now any point ω in the complement of ''O'' is such that lim ''Xn''(ω) = ''X''(ω), which implies that |''Xn''(ω) − ''X''(ω)| < ε for all ''n'' greater than a certain number ''N''. Therefore, for all ''n'' ≥ ''N'' the point ω will not belong to the set ''An'', and consequently it will not belong to ''A''. This means that ''A'' is disjoint with ''O'', or equivalently, ''A'' is a subset of ''O'' and therefore Pr(''A'') = 0.
Finally, consider
: \operatorname\left(|X_n-X|>\varepsilon\right) \leq \operatorname(A_n) \ \underset 0,
which by definition means that ''Xn'' converges in probability to ''X''.

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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