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,…,Xn⊆Ω.
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 X−1(B) is Σ-measurable for every Borel set B, the disjoint collection {X−1({a})}a∈R must have at most finitely many (≤2M) elements, thus we are done.
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