Definition:Even Integer/Definition 2

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An integer $n \in \Z$ is even if and only if it is of the form:

$n = 2 r$

where $r \in \Z$ is an integer.

Also see

Historical Note

The concept of classifying numbers as odd or even appears to have originated with the Pythagoreans.

It was their belief that even numbers are female, and odd numbers are male.

A commentator on Plato used the term isosceles number for an even number, in correspondence with the concept of an isosceles triangle. In a similar way an odd number was described as scalene.