The aim of this lecture is to present a few examples of number systems
that occur in computer arithmetic. We will illustrate how relevant
some number systems, or some changes of representations are, for an
efficient computation of some operations or function evaluations.
We will consider in particular signed digit number systems: Booth
and NAF, as well as redundant ones applied to CORDIC algorithms. Then, we will
present some modular number systems well suited for large numbers
and cryptography.