Horizontal Section of Simple Function is Simple Function
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Theorem
Let $\struct {X, \Sigma_X}$ and $\struct {Y, \Sigma_Y}$ be measurable spaces.
Let $\struct {X \times Y, \Sigma_X \otimes \Sigma_Y}$ be the product measurable space of $\struct {X, \Sigma_X}$ and $\struct {Y, \Sigma_Y}$.
Let $f : X \times Y \to \R$ be a simple function.
Let $y \in Y$.
Then $f^y : X \to \R$ is a simple function, where $f^y$ is the $y$-horizontal section of $f$.
Proof
Write the standard representation of $f$ as:
- $\ds f = \sum_{k \mathop = 1}^n a_k \chi_{E_k}$
with:
- $E_1, E_2, \ldots, E_n$ pairwise disjoint $\Sigma_X \otimes \Sigma_Y$-measurable sets
- $a_1, a_2, \ldots, a_n$ real numbers.
We have:
\(\ds f^y\) | \(=\) | \(\ds \paren {\sum_{k \mathop = 1}^n a_k \chi_{E_k} }^y\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 1}^n a_k \paren {\chi_{E_k} }^y\) | Horizontal Section of Linear Combination of Functions is Linear Combination of Horizontal Sections | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 1}^n a_k \chi_{\paren {E_k}^y}\) | Horizontal Section of Characteristic Function is Characteristic Function of Horizontal Section |
From Intersection of Horizontal Sections is Horizontal Section of Intersection, we have that:
- $\paren {E_1}^y, \paren {E_2}^y, \ldots, \paren {E_n}^y$ are pairwise disjoint.
From Horizontal Section of Measurable Set is Measurable, we have that:
- $\paren {E_1}^y, \paren {E_2}^y, \ldots, \paren {E_n}^y$ are $\Sigma_X$-measurable.
So, we have:
- $\ds f^y = \sum_{k \mathop = 1}^n a_k \chi_{\paren {E_k}^y}$
with:
- $\paren {E_1}^y, \paren {E_2}^y, \ldots, \paren {E_n}^y$ pairwise disjoint $\Sigma_X$-measurable sets
- $a_1, a_2, \ldots, a_n$ real numbers.
So $f^y$ is a simple function.
$\blacksquare$