\begin{definition}[Definition:Positive/Integer]
Informally, the '''positive integers''' are the set:
:$\Z_{\ge 0} = \set {0, 1, 2, 3, \ldots}$
As the set of integers $\Z$ is the Inverse Completion of Natural Numbers, it follows that elements of $\Z$ are the isomorphic images of the elements of equivalence classes of $\N \times \N$ where two tuples are equivalent if the difference between the two elements of each tuple is the same.
Thus '''positive''' can be formally defined on $\Z$ as a relation induced on those equivalence classes as specified in the definition of integers.
That is, the integers being defined as all the difference congruence classes, '''positive''' can be defined directly as the relation specified as follows:
:The integer $z \in \Z: z = \eqclass {\tuple {a, b} } \boxminus$ is '''positive''' {{iff}} $b \le a$.
The set of '''positive integers''' is denoted $\Z_{\ge 0}$.
An element of $\Z$ can be specifically indicated as being '''positive''' by prepending a $+$ sign:
:$+x := x \in \Z_{\ge 0}$.
\end{definition}