Regular value

转自:http://en.wikipedia.org/wiki/Critical_point_(mathematics)

For a differentiable map f between R^m and Rⁿ, critical points are the points where the differential of f is a linear map of rank less than n; in particular, every point is critical if m < n. This definition immediately extends to maps between smooth manifolds. The image of a critical point under f is a called a critical value. A point in the complement of the set of critical values is called a regular value.