Horizontal Section preserves Subsets
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Theorem
Let $X$ and $Y$ be sets.
Let $A \subseteq B$ be subsets of $X \times Y$.
Let $y \in Y$.
Then:
- $A^y \subseteq B^y$
where $A^y$ is the $y$-horizontal section of $A$ and $B^y$ is the $y$-horizontal section of $B$.
Proof
Note that if:
- $x \in A^y$
from the definition of $x$-vertical section, we have:
- $\tuple {x, y} \in A$
so:
- $\tuple {x, y} \in B$
So, from the definition of $x$-vertical section, we have:
- $x \in B^y$
So:
- if $x \in A^y$ then $x \in B^y$.
That is:
- $A^y \subseteq B^y$
$\blacksquare$