A <em>natural number</em> is an element of the set
\NN={0,1,2,...}.

An <em>integer</em> is either a natural number, or its negative.
The set of integers is \ZZ.

One can complete \ZZ to the set \QQ of <em>rational numbers</em>
by introducing fractions of integers.

Then one completes \QQ to the set \RR of <em>real numbers</em>
by introducing finite limits.

And \RR is completed to \CC, the set of <em>complex numbers</em>,
by adding roots of polynomials.

Conclusion:<br />
<div class="wimscenter">
 \NN \subset \ZZ \subset \QQ \subset \RR \subset \CC
</div>
