# Equivalence Relation/Examples/Months that Start on the Same Day of the Week

## Contents

## Example of Equivalence Relation

Let $M$ be the set of months of the year according to the (usual) Gregorian calendar.

Let $\sim$ be the relation on $M$ defined as:

- $\forall x, y \in M: x \sim y \iff \text {$x$ and $y$ both start on the same day of the week}$

Then $\sim$ is an equivalence relation.

## Proof

Checking in turn each of the criteria for equivalence:

### Reflexivity

Let $x \in M$.

Then $x$ starts on the same day of the week as itself.

Thus $\sim$ is seen to be reflexive.

$\Box$

### Symmetry

Let $x, y \in M$.

If $x$ starts on the same day of the week as $y$, then $y$ starts on the same day of the week as $x$.

Thus $\sim$ is seen to be symmetric.

$\Box$

### Transitivity

Let $x, y, z \in M$.

Let $x$ start on the same day of the week as $y$.

Let $y$ start on the same day of the week as $z$.

Then $x$ starts on the same day of the week as $z$

Thus $\sim$ is seen to be transitive.

$\Box$

$\sim$ has been shown to be reflexive, symmetric and transitive.

Hence by definition it is an equivalence relation.

$\blacksquare$

## Sources

- 1975: T.S. Blyth:
*Set Theory and Abstract Algebra*... (previous) ... (next): $\S 6$. Indexed families; partitions; equivalence relations: Exercise $7$