# measurable function

Let $\big{(}X,\mathcal{B}(X)\big{)}$ and $\big{(}Y,\mathcal{B}(Y)\big{)}$ be two measurable spaces   . Then a function $f\colon X\to Y$ is called a measurable function  if:

 $f^{-1}\big{(}\mathcal{B}(Y)\big{)}\subseteq\mathcal{B}(X)$

where $f^{-1}\big{(}\mathcal{B}(Y)\big{)}=\{f^{-1}(E)\mid E\in\mathcal{B}(Y)\}$.

In other words, the inverse image of every $\mathcal{B}(Y)$-measurable set is $\mathcal{B}(X)$-measurable. The space of all measurable functions $f\colon X\to Y$ is denoted as

 $\mathcal{M}\big{(}\big{(}X,\mathcal{B}(X)\big{)},\big{(}Y,\mathcal{B}(Y)\big{)% }\big{)}.$

Any measurable function into $(\mathbb{R},\mathcal{B}(\mathbb{R}))$, where $\mathcal{B}(\mathbb{R})$ is the Borel sigma algebra of the real numbers $\mathbb{R}$, is called a Borel measurable function.11More generally, a measurable function is called Borel measurable if the range space $Y$ is a topological space  with $\mathcal{B}(Y)$ the sigma algebra generated by all open sets of $Y$. The space of all Borel measurable functions from a measurable space $(X,\mathcal{B}(X))$ is denoted by $\displaystyle{\mathcal{L}^{0}\big{(}X,\mathcal{B}(X)\big{)}}$.

Similarly, we write $\displaystyle{\bar{\mathcal{L}}^{0}\big{(}X,\mathcal{B}(X)\big{)}}$ for $\displaystyle{\mathcal{M}\big{(}\big{(}X,\mathcal{B}(X)),(\bar{\mathbb{R}},% \mathcal{B}(\bar{\mathbb{R}})\big{)}\big{)}}$, where $\mathcal{B}(\bar{\mathbb{R}})$ is the Borel sigma algebra of $\bar{\mathbb{R}}$, the set of extended real numbers.

Remark. If $f:X\to Y$ and $g:Y\to Z$ are measurable functions, then so is $g\circ f:X\to Z$, for if $E$ is $\mathcal{B}(Z)$-measurable, then $g^{-1}(E)$ is $\mathcal{B}(Y)$-measurable, and $f^{-1}\big{(}g^{-1}(E)\big{)}$ is $\mathcal{B}(X)$-measurable. But $f^{-1}\big{(}g^{-1}(E)\big{)}=(g\circ f)^{-1}(E)$, which implies that $g\circ f$ is a measurable function.

Example:

 Title measurable function Canonical name MeasurableFunction Date of creation 2013-03-22 12:50:50 Last modified on 2013-03-22 12:50:50 Owner CWoo (3771) Last modified by CWoo (3771) Numerical id 18 Author CWoo (3771) Entry type Definition Classification msc 28A20 Synonym Borel measurable Related topic ExampleOfFunctionNotLebesgueMeasurableWithMeasurableLevelSets Related topic LusinsTheorem2 Related topic BorelGroupoid Related topic BorelMorphism Defines Borel measurable function