martingale proof of Kolmogorov’s strong law for square integrable variables
We apply the martingale convergence theorem to prove the following result.
Theorem.
Let X1,X2,… be independent random variables such that ∑nVar[Xn]/n2<∞. Then, setting
Sn=1nn∑k=1(Xk-𝔼[Xk]) |
we have Sn→0 as n→∞, with probability one.
To prove this, we start by constructing a martingale,
Mn=n∑k=1Xk-𝔼[Xk]k. |
If ℱn is the σ-algebra (http://planetmath.org/SigmaAlgebra) generated by X1,…Xn then
𝔼[Mn+1∣ℱn]=Mn+𝔼[Xn+1∣ℱn]-𝔼[Xn+1]n+1=Mn. |
Here, the independence of Xn+1 and ℱn has been used to imply that 𝔼[Xn+1∣ℱn]=𝔼[Xn+1]. So, M is a martingale with respect to the filtration (ℱn)n∈ℕ.
Also, by the independence of the Xn, the variance of Mn is
Var[Mn]=n∑k=1Var[Xk/k]≤∞∑k=1Var[Xk]k2<∞. |
So, the inequality 𝔼[|Mn|]≤√𝔼[M2n]=√Var[Mn] shows that M is an L1-bounded martingale, and the martingale convergence theorem says that the limit M∞=lim exists and is finite, with probability one.
The strong law now follows from Kronecker’s lemma, which states that for sequences of real numbers and such that strictly increases to infinity
and converges to a finite limit, then tends to as . In our case, we take and to deduce that converges to zero with probability one.
References
- 1 David Williams, Probability with martingales, Cambridge Mathematical Textbooks, Cambridge University Press, 1991.
- 2 Olav Kallenberg, Foundations of modern probability, Second edition. Probability and its Applications. Springer-Verlag, 2002.
Title | martingale proof of Kolmogorov’s strong law for square integrable variables |
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Canonical name | MartingaleProofOfKolmogorovsStrongLawForSquareIntegrableVariables |
Date of creation | 2013-03-22 18:33:51 |
Last modified on | 2013-03-22 18:33:51 |
Owner | gel (22282) |
Last modified by | gel (22282) |
Numerical id | 4 |
Author | gel (22282) |
Entry type | Proof |
Classification | msc 60F15 |
Classification | msc 60G42 |
Related topic | MartingaleConvergenceTheorem |
Related topic | KolmogorovsStrongLawOfLargeNumbers |
Related topic | StrongLawOfLargeNumbers |
Related topic | ProofOfKolmogorovsStrongLawForIIDRandomVariables |