Equivalence of Definitions of Right Quasi-Reflexive Relation
Theorem
Let $\RR \subseteq S \times S$ be a relation in $S$.
The following definitions of the concept of Right Quasi-Reflexive Relation are equivalent:
Definition 1
$\RR$ is right quasi-reflexive if and only if:
- $\forall x, y \in S: \tuple {x, y} \in \RR \implies \tuple {y, y} \in \RR$
Definition 2
$\RR$ is right quasi-reflexive if and only if:
- $\forall y \in \Img \RR: \tuple {y, y} \in \RR$
where $\Img \RR$ denotes the image set of $\RR$.
Proof
$(1)$ implies $(2)$
Let $\RR$ be a right quasi-reflexive relation by definition $1$.
Then by definition:
- $\forall x, y \in S: \tuple {x, y} \in \RR \implies \tuple {y, y} \in \RR$
Let $y \in \Img \RR$ be arbitrary.
Then by definition of image set:
- $\exists x \in S: \tuple {x, y} \in \RR$
Hence by hypothesis:
- $\tuple {y, y} \in \RR$
As $y$ is arbitrary:
- $\forall y \in \Img \RR: \tuple {y, y} \in \RR$
Thus $\RR$ is a right quasi-reflexive relation by definition $2$.
$\Box$
$(2)$ implies $(1)$
Let $\RR$ be a right quasi-reflexive relation by definition $2$.
Then by definition:
- $\forall y \in \Img \RR: \tuple {y, y} \in \RR$
Let $x, y \in S$ be arbitrary such that $\tuple {x, y} \in \RR$.
Then by definition of image set:
- $y \in \Img \RR$
Hence by hypothesis:
- $\tuple {y, y} \in \RR$
As $\tuple {x, y} \in \RR$ is arbitrary:
- $\forall x, y \in S: \tuple {x, y} \in \RR \implies \tuple {y, y} \in \RR$
Thus $\RR$ is a right quasi-reflexive relation by definition $1$.
$\blacksquare$