Measurable Function

Definition (Measurable)

Let (X,FX),(Y,FY)(X,\mathcal{F}_{X}),(Y,\mathcal{F}_{Y}) be measurable spaces. Let g:(X,FX)(Y,FY)g:(X,\mathcal{F}_{X})\to(Y,\mathcal{F}_{Y}) be a mapping. gg is called a measurable function if AFY:g1(A)FX\forall A\in\mathcal{F}_{Y}:g^{-1}(A)\in\mathcal{F}_{X}i.e. the pre-images of all sets in the output σ-algebra are in the σ-algebra of the input space.


In the context of our analysis class we can examine this definition from the POV of topological spaces

Let (X,M)(X,\mathcal{M}) be a measurable space and (Y,T)(Y,\mathscr{T}) be a topological space. We say f:XYf:X\to Y is measurable if BT:f1(B)M\forall B\in\mathscr{T}:f^{-1}(B)\in\mathcal{M}

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