Reflexive Relation is Quasi-Reflexive
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Theorem
Let $\RR$ be a reflexive relation on a set $S$.
Then $\RR$ is a quasi-reflexive relation on $S$.
Proof
By definition of reflexive relation:
- $\forall x \in S: \tuple {x, x} \in \RR$
Hence by definition of domain:
- $x \in \Dom \RR$
and hence by definition of field and Set is Subset of Union:
- $x \in \Field \RR$
That is:
- $\forall x \in \Field \RR: \tuple {x, x} \in \RR$
Hence the result by definition of quasi-reflexive relation.
$\blacksquare$