# Equivalence of Definitions of Reflexive Relation

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## Theorem

The following definitions of the concept of Reflexive Relation are equivalent:

### Definition 1

$\RR$ is reflexive if and only if:

$\forall x \in S: \tuple {x, x} \in \RR$

### Definition 2

$\RR$ is reflexive if and only if it is a superset of the diagonal relation:

$\Delta_S \subseteq \RR$

## Proof

### Definition 1 implies Definition 2

Let $\RR$ be a relation.

We use a Proof by Contraposition by showing that:

$\Delta_S \not \subseteq \RR \implies \exists x \in S: \tuple {x, x} \notin \RR$

Thus, suppose:

$\Delta_S \not \subseteq \RR$

Then by definition of Diagonal Relation:

$\exists \tuple {x, x} \in S \times S: \tuple {x, x} \notin \RR$

Thus:

$\exists x \in S: \tuple {x, x} \notin \RR$

From Rule of Transposition it follows that:

$\forall x \in S: \tuple {x, x} \in \RR \implies \Delta_S \subseteq \RR$

$\Box$

### Definition 2 implies Definition 1

Let $\RR$ be a relation which fulfils the condition:

$\Delta_S \subseteq \RR$

Then:

 $\displaystyle \forall x \in S: \tuple {x, x}$ $\in$ $\displaystyle \Delta_S$ Definition of Diagonal Relation $\displaystyle \leadsto \ \$ $\displaystyle \forall x \in S: \tuple {x, x}$ $\in$ $\displaystyle \RR$ Definition of Subset

Thus $\RR$ is reflexive by Definition 1.

$\blacksquare$