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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>