Definition:Integer

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Informal Definition

The numbers $\left\{{\ldots, -3, -2, -1, 0, 1, 2, 3, \ldots}\right\}$ are called the integers.

They are also referred to as whole numbers, so as to distinguish them from fractions. However, use of this term is discouraged because it is ambiguous: it can refer to the integers, the positive integers, or the negative integers, depending on the preference of the author.


This set is usually denoted $\Z$ (Z for Zahlen, which is German for whole numbers, with overtones of unbroken).

Variants on $\Z$ are often seen, for example $\mathbf Z$ and $\mathcal Z$, or even just $Z$.

Some sources use $\mathbf J$ or a variant.


An individual element of $\Z$ is called an integer.


Formal Definition

Let $\left ({\N, +}\right)$ be the commutative semigroup of natural numbers under addition.

From Inverse Completion of Natural Numbers, we can create $\left({\N', +'}\right)$, an inverse completion of $\left ({\N, +}\right)$.

From Construction of Inverse Completion, this is done as follows:


Let $\boxminus$ be the congruence relation defined on $\N \times \N$ by:

$\left({x_1, y_1}\right) \boxminus \left({x_2, y_2}\right) \iff x_1 + y_2 = x_2 + y_1$

The fact that this is a congruence relation is proved in Equivalence Relation on Semigroup Product with Cancellable Elements.


Let $\left({\N \times \N, \oplus}\right)$ be the external direct product of $\left({\N, +}\right)$ with itself, where $\oplus$ is the operation on $\N \times \N$ induced by $+$ on $\N$:

$\left({x_1, y_1}\right) \oplus \left({x_2, y_2}\right) = \left({x_1 + x_2, y_1 + y_2}\right)$

Let the quotient structure defined by $\boxminus$ be $\displaystyle \left({\frac {\N \times \N} {\boxminus}, \oplus_{\boxminus}}\right)$

where $\oplus_{\boxminus}$ is the operation induced on $\displaystyle \frac {\N \times \N} \boxminus$ by $\oplus$.

Let us use $\N'$ to denote the quotient set $\displaystyle \frac {\N \times \N} {\boxminus}$.

Let us use $+'$ to denote the operation $\oplus_{\boxminus}$.


Thus $\left({\N', +'}\right)$ is the Inverse Completion of Natural Numbers.

As the Inverse Completion is Unique up to isomorphism, it follows that we can define the structure $\left({\Z, +}\right)$ which is isomorphic to $\left({\N', +'}\right)$.

An element of $\N'$ is therefore an equivalence class of the congruence relation $\boxminus$.

So an element of $\Z$ is the isomorphic image of an element $\left[\!\left[{\left({a, b}\right)}\right]\!\right]_\boxminus$ of $\displaystyle \frac {\N \times \N} \boxminus$.


The set of elements $\Z$ is called the integers.


From the comment in the proof of Construction of Inverse Completion: This Equivalence Relation is a Congruence, it can be seen that the equivalence classes which are the elements of $\Z$ can be characterized by identifying each class with the difference.


Pronunciation

The word integer is pronounced with the stress on the first syllable, and the g is soft (i.e. sounds like j).


Notation

Note that $\left[\!\left[{\left({a, b}\right)}\right]\!\right]_\boxminus$ is an equivalence class of ordered pairs of natural numbers under the congruence relation $\boxminus$.

As this notation is cumbersome, it is commonplace though technically incorrect to streamline it to $\left[\!\left[{a, b}\right]\!\right]_\boxminus$, or $\left[\!\left[{a, b}\right]\!\right]$.

This is generally considered acceptable, as long as it is made explicit as to the precise meaning of $\left[\!\left[{a, b}\right]\!\right]$ at the start of any exposition.


Sources

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