Horizontal Section preserves Pointwise Limits of Sequences of Functions
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Theorem
Let $X$ and $Y$ be sets.
Let $f : X \times Y \to \overline \R$ be a function.
Let $\sequence {f_n}_{n \mathop \in \N}$ be a sequence of functions converging pointwise to $f$.
Let $y \in Y$.
Then:
- $\paren {f_n}^y \to f^y$
pointwise, where:
- $\paren {f_n}^y$ denotes the $y$-horizontal section of $f_n$
- $f^y$ denotes the $y$-horizontal section of $f$.
Proof
From the definition of pointwise convergence, we have:
- $\ds \map f {x, y} = \lim_{n \mathop \to \infty} \map {f_n} {x, y}$
for each $x \in X$ and $y \in Y$.
Fix $y \in Y$.
From the definition of the $y$-horizontal section, we have:
- $\map {f_n} {x, y} = \map {\paren {f_n}^y} x$
and:
- $\map f {x, y} = \map {f^y} x$
So:
- $\ds \map {f^y} x = \lim_{n \mathop \to \infty} \map {\paren {f_n}^y} x$
for each $x \in X$.
So:
- $\paren {f_n}^y \to f^y$
$\blacksquare$