A function 
has critical points at all points 
where 
or 
is not differentiable.
A function 
has critical points where the gradient 
or 
or the partial
derivative 
is not defined.
Critical Point
See also
Fixed Point, Inflection Point, Only Critical Point in Town Test, Stationary Point Explore this topic in the MathWorld classroomExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Critical Point." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/CriticalPoint.html