An *arithmetic sequence* is a list of numbers where the
numbers go up (or down) by the same amount each time. For example,
3, 5, 7, 9 make an arithmetic sequence because they go up by 2
each time. Similarly, 4, 1, −2, −5, −8 is
arithmetic, since they go down by 3 each time. A special case is a
sequence like 4, 4, 4, 4, 4, 4, 4. This is also arithmetic, since
it goes up by 0 each time.