Perpendicularity is Antireflexive Relation
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Theorem
Let $S$ be the set of straight lines in the plane.
For $l_1, l_2 \in S$, let $l_1 \perp l_2$ denote that $l_1$ is perpendicular to $l_2$.
Then $\perp$ is an antireflexive relation on $S$.
Proof
By definition of perpendicular lines, for $l_1$ to be perpendicular to itself would mean it would have to meet itself in a right angle.
This it does not do.
So $l_1 \not \perp l_1$.
Thus $\perp$ is seen to be antireflexive.
$\blacksquare$
Also see
Sources
- 1964: Steven A. Gaal: Point Set Topology ... (previous) ... (next): Introduction to Set Theory: $1$. Elementary Operations on Sets