Definition: We say that a number $f(b)$ is a maximum value
for $f$ on a set D, if whenever $a \in D$ , $f(a) \le f(b)$.
We say that a number $f(b)$ is a minimum value for $f$ on a
set D, if whenever $a \in D$ , $f(a) \ge f(b)$.
We say that a number $f(b)$ is an extreme value for $f$ on a
set D, if $f(b)$ is a maximum value for $f$ on a
set D or $f(b)$ is a minimum value for $f$ on a set
D
