Produced by Charles Wells Revised 2014 1116 Introduction to this website website TOC website index blog
An integer is any whole number. An integer can be zero, greater than zero or less than zero. The integers greater than zero are the natural numbers. I will not give a formal definition of integer here but the Wikipedia article gives one. This article describes their most elementary properties and points out some sources of confusion.
If $m$ and $n$ are integers, then so are $m+n$, $m–n$ and $mn.$
This is described by saying that the integers are closed under addition, subtraction and multiplication. They are not closed under division. For example, $3$ and $5$ are integers but $3/5$ is not an integer.
For any integer $n$:
Some things to pay attention to:
In some countries, schools may use “positive” to mean “nonnegative”, so that zero is positive. In American mathematical usage, $0$ is neither positive or negative. Occasionally, students have told me that their high school teachers said $0$ is both positive and negative.
I will give two ways it is useful to think about integers. You will no doubt be familiar with them. They are included here partly to illustrate the ways in which we think about mathematical objects using images and metaphors. (In other words, in this section, I am not teaching you about the integers as much as I am teaching you about images and metaphors!)
The positive integers increase in size to the right and the negative integers increase in size to the left:
In contrast to the natural numbers they form a list that is infinite in both directions.
Adding a positive number n to another one takes you to the number n spaces to the right. Adding a negative number –n takes you to the number n spaces to the left.
A negative integer such as $-5$ does not have an immediately obvious interpretation as the number of elements of a set. One interpretation that does make sense of negative integers involves credit and debit. If you have $n$ dollars (for $n$ positive) that is what you have. Having $-n$ dollars represents the fact that you owe $n$ dollars.