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If x 0 is an interior point in the domain of a function f, then f is said to be differentiable at x 0 if the derivative ′ exists. In other words, the graph of f has a non-vertical tangent line at the point (x 0, f(x 0)). f is said to be differentiable on U if it is differentiable at every point of U.
A function is differentiable at an interior point a of its domain if and only if it is semi-differentiable at a and the left derivative is equal to the right derivative. An example of a semi-differentiable function, which is not differentiable, is the absolute value function () = | |, at a = 0.
Even a function with a smooth graph is not differentiable at a point where its tangent is vertical: For instance, the function given by () = / is not differentiable at =. In summary, a function that has a derivative is continuous, but there are continuous functions that do not have a derivative. [13]
This function is continuous on the closed interval [−r, r] and differentiable in the open interval (−r, r), but not differentiable at the endpoints −r and r. Since f (−r) = f (r), Rolle's theorem applies, and indeed, there is a point where the derivative of f is zero. The theorem applies even when the function cannot be differentiated ...
A critical point of a function of a single real variable, f (x), is a value x 0 in the domain of f where f is not differentiable or its derivative is 0 (i.e. ′ =). [2] A critical value is the image under f of a critical point.
However, this function is not continuously differentiable. A smooth function that is not analytic. The function = {, < is continuous, but not differentiable at x = 0, so it is of class C 0, but not of class C 1.
Despite being differentiable nowhere, the function is continuous: Since the terms of the infinite series which defines it are bounded by and this has finite sum for < <, convergence of the sum of the terms is uniform by the Weierstrass M-test with =.
In other words, the value of the constant function, y, will not change as the value of x increases or decreases. At each point, the derivative is the slope of a line that is tangent to the curve at that point. Note: the derivative at point A is positive where green and dash–dot, negative where red and dashed, and zero where black and solid.