An Equivalence Relation is a binary relation $\equiv$ on a set A which has the following properties:
For all a, b and c $\epsilon$ A
- Transitivity: if $a\equiv{b}$ and $b\equiv{c}$ then $a\equiv{c}$
- Symmetry: if $a\equiv{b}$ then $b\equiv{a}$
- Reflexivity: $a\equiv{a}$
Once you have an equivalance relation on a set, you can take one element (say, x) and look at all
the elements that are equivalent to it. This is called the equivalence class of x.
- $E(x)=\{y\in{A}:~y{\equiv}x~\}$
It's clear that a finite set can be divided up into a finite number of non-empty, disjoint
sets, each of which is an equivalence class (of some element). Most people would agree that
a countably infinite set can also be divided up into equivalence classes.
It's less clear that dividing up an uncountably infinite set can always be accomplished.
This has a connection with the Axiom of Choice.
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