Posted 6 November
2008

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,
but many of their basic properties will be spelled out here and in the
chapter on number theory.
The name “number theory” can cause cognitive dissonance: it sounds as if number theory should be the theory of all numbers!

Examples 
Terminology Number theory (MW, Wik) is the branch of math concerned with the theory of the natural numbers. Abstractmath.org has a separate chapter introducing some basic ideas about number theory.
If m and n are integers, then so are m+n, mn abd 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.
Compare:
¨ The natural numbers are closed under addition and multiplicatrion but not subtraction.
¨ The rational numbers are closed under addition, subtraction, multiplication and division by a nonzero rational.
For any integer n:
Terminology In
France and possibly other countries schools may use “positive” to mean
“nonnegative”, so that zero is positive. In abstractmath.org,
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. This is a special case of the way positive and negative distances are represented on the real line. (Warning: See minus.)
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.
Credit and debit for substances requires the real numbers.