Which of the following statements is FALSE for a differentiable function f(x)?
1
At a critical point of f(x), the derivative of f(x) is either 0 or undefined.
2
A critical point of f(x) must belong to the domain of f(x).
3
f(x) has at least one critical point.
4
A local extremum of f(x) must occur at a critical point of f(x).