Equivalence of Definitions of Reflexive Relation

Definition 1 implies Definition 2
Let $\mathcal R$ be a relation.

We use a Proof by Contraposition by showing that:


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

Thus, suppose:
 * $\Delta_S \not \subseteq \mathcal R$

Then by definition of Diagonal Relation:
 * $\exists \tuple {x, x} \in S \times S: \tuple {x, x} \notin \mathcal R$

Thus:
 * $\exists x \in S: \tuple {x, x} \notin \mathcal R$

From Rule of Transposition it follows that:
 * $\forall x \in S: \tuple {x, x} \in \mathcal R \implies \Delta_S \subseteq \mathcal R$

Definition 2 implies Definition 1
Let $\mathcal R$ be a relation which fulfils the condition:
 * $\Delta_S \subseteq \mathcal R$

Then:

Thus $\mathcal R$ is reflexive by Definition 1.