It frequently happens that we prefer to ignore multiples of a given number
when we do calculations. Just think of the days in a week or the hours in
a day; in the first case we ignore multiples of seven, in the second case
multiples of 24.
In this chapter we will describe this `arithmetic modulo n'.
It will prove to be useful in the discussion of
number systems distinct
from the decimal system.
Other applications are the
RSA cryptosystem
and a radar detection pattern.