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Thursday, May 7, 2015

Record an observation

Let Ω be a measure space, a σ-algebra of subsets of Ω is said to be finite if it is generated by finitely many subsets X1,,XnΩ.

Observation. A random variable X on Ω is simple if and only if it is measurable w.r.t. a finite σ-algebra.

Proof. Suppose that X=ni=1xiIAi, then the σ-algebra induced by X is precisely σ(A1,,An), which is finite. Conversely, suppose that X is measurable on Σ=σ(B1,,Bn), then every element in σ(B1,,Bn) is a finite union of elements in some measurable partition C={C1,,CM}. Moreover, since X1(B) is Σ-measurable for every Borel set B, the disjoint collection {X1({a})}aR must have at most finitely many (2M) elements, thus we are done.

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