# Equivalence relation and equivalence classes given function and relation

Given a function $f : A → B$, let $R$ be the relation defined on $A$ by $aRa′$ whenever $f(a) = f(a′)$. Prove that $R$ is an equivalence relation and determine the equivalence classes.

To prove that $R$ is an equivalence relation, I know I have to show that R is reflexive, symmetric, and transitive. And from there, I can also determine the equivalence classes. However, I'm not sure where exactly to start. What exactly is the relation?

• $R \subseteq A \times A : (a,b) \in R$ iff $f(a)=f(b)$. Thus, e.g. for reflexivity, $(a,a) \in R$ because, obviously: $f(a)=f(a)$. – Mauro ALLEGRANZA Apr 24 '16 at 17:17
• As a matter of fact, this is the easiest and also only example of equivalence relations – Hagen von Eitzen Apr 24 '16 at 17:20

## 2 Answers

$f(a) = f(a) \implies R$ is reflexive, and if $aRb \implies f(a) = f(b) \implies f(b) = f(a) \implies bRa$, hence $R$ is symmetric. And if $aRb, bRc \implies f(a) = f(b), f(b) = f(c) \implies f(a) = f(c) \implies aRc$. Thus $R$ is equivalence relation on $A$. The equivalent classes consist of the $f^{-1}(x)'s$ whereas $x \in$ the range of $f$.

• How can we tell that the equivalent classes consist of that? – discsq12 Apr 24 '16 at 22:44

It is equivalent to speak about an equivalence relation $R$ on a set $A$ and a partition of $A$ into its classes of equivalence.

Hence there is a natural partition by the preimages $f^{-1}(b)$ for all $b \in B$ (which are either disjoint or identical), in other terms:

$$aRa' \Leftrightarrow f(a)=f(a') \Leftrightarrow a \ \text{and} \ a' \ \text{belong to the same preimage}$$