TheoremBase

A Function Nondecreasing on a Borel Subset of the Real Line and Vanishing Outside It is Borel

If a real function is nondecreasing on a Borel subset of the real line and vanishes off that subset, then it is Borel measurable.

Statement

In the setting of The Real Numbers: Standing Notation and Background, let B(R)\mathcal{B}(\mathbb{R}) be the Borel σ\sigma-algebra of the real line, and let measurable mean measurable with respect to B(R)\mathcal{B}(\mathbb{R}) and B(R)\mathcal{B}(\mathbb{R}).

Let E∈B(R)E\in\mathcal{B}(\mathbb{R}) and let T:R→RT:\mathbb{R}\to\mathbb{R}.

1. (Borel measurability) Suppose that T(x)≤T(x′)T(x)\le T(x') for all x,x′∈Ex,x'\in E with x≤x′x\le x', and that T(x)=0T(x)=0 for every x∈R∖Ex\in\mathbb{R}\setminus E. Then TT is measurable.

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