# Symmetric Transitive and Serial Relation is Reflexive

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## Theorem

Let $\mathcal R$ be a relation which is:

symmetric
transitive
serial.

Then $\mathcal R$ is reflexive.

Thus such a relation is an equivalence.

## Proof

Let $S$ be a set on which $\mathcal R$ is a relation which is symmetric, transitive and serial.

As $\mathcal R$ is symmetric:

$x \mathcal R y \implies y \mathcal R x$

As $\mathcal R$ is transitive:

$x \mathcal R y \land y \mathcal R x \implies x \mathcal R x$

As $\mathcal R$ is serial:

$\forall x \in S: \exists y \in S: x \mathcal R y$

Let $x \in S$.

Then

 $\, \displaystyle \exists y \in S: \,$ $\displaystyle \left({x, y}\right)$ $\in$ $\displaystyle \mathcal R$ as $\mathcal R$ is serial $\displaystyle \implies \ \$ $\displaystyle \left({y, x}\right)$ $\in$ $\displaystyle \mathcal R$ as $\mathcal R$ is symmetric $\displaystyle \implies \ \$ $\displaystyle \left({x, x}\right)$ $\in$ $\displaystyle \mathcal R$ as $\mathcal R$ is transitive

Thus:

$\forall x: x \mathcal R x$

and by definition $\mathcal R$ is reflexive.

It follows by definition that such a relation is an equivalence relation.

$\blacksquare$