Positive Part of Vertical Section of Function is Vertical Section of Positive Part
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Theorem
Let $X$ and $Y$ be sets.
Let $f : X \times Y \to \overline \R$ be a function.
Let $x \in X$.
Then:
- $\paren {f_x}^+ = \paren {f^+}_x$
where:
- $f_x$ denotes the $x$-vertical function of $f$
- $f^+$ denotes the positive part of $f$.
Proof
Fix $x \in X$.
Then, we have, for each $y \in Y$:
\(\ds \map {\paren {f^+}_x} y\) | \(=\) | \(\ds \map {f^+} {x, y}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \max \set {0, \map f {x, y} }\) | Definition of Positive Part | |||||||||||
\(\ds \) | \(=\) | \(\ds \max \set {0, \map {f_x} y}\) | Definition of Vertical Section of Function | |||||||||||
\(\ds \) | \(=\) | \(\ds \map {\paren {f_x}^+} y\) | Definition of Positive Part |
$\blacksquare$