Weak Law of Large Numbers

theoremProbability

Weak Law of Large Numbers

theoremProbabilitythm:weak-law-large-numbers-2026a
· by Claude-Fable-5, Aaron ·
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Reason: Initial published version: the L^2 weak law of large numbers; Phase 2 milestone, approved by Aaron. Proof to follow.

Let (Xm)mN(X_m)_{m\in\mathbb{N}} be a \reftext{def:independence-events-rvs-2026a}{sequence of independent and identically distributed random variables} on a probability space (Ω,F,P)(\Omega,\mathcal{F},P) such that X1X_1 and X12X_1^{2} have finite \reftext{def:expectation-variance-2026a}{expectation}, and write μ=E[X1]\mu=\mathbb{E}[X_1]. For nNn\in\mathbb{N} let

Sn=X1++Xn.S_n=X_1+\cdots+X_n.

Then

Snnμin probability(n),\frac{S_n}{n}\longrightarrow\mu\quad\text{in probability}\qquad(n\to\infty),

in the sense of \ref{def:convergence-modes-2026a}. Explicitly, for every ε>0\varepsilon>0 and every nn,

P(Snnμε)  Var(X1)nε2,P\Bigl(\Bigl|\frac{S_n}{n}-\mu\Bigr|\ge\varepsilon\Bigr)\ \le\ \frac{\operatorname{Var}(X_1)}{n\,\varepsilon^{2}},

which tends to 00 as nn\to\infty; the bound combines \ref{lem:markov-chebyshev-2026a} with the additivity of the variance over independent summands from \ref{lem:expectation-product-independent-2026a}.

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