The most important example of a ring is the set Z of integers, with the usual
addition and multiplication. The various properties should be familiar to you; we
will simply accept that they hold. Z is a commutative ring with identity. It is not a
division ring because there is no integer b satisfying 2b = 1. This ring will be our
prototype for several things in the course.
Note that the set N of natural numbers, or non-negative integers, is not a ring,
since it fails the inverse law for addition. (There is no non-negative integer b such
that 2+b = 0.)