# example of a function that is continuous but not differentiable

• Apr 22nd 2010, 05:22 PM
dgmath
example of a function that is continuous but not differentiable
I need an example of a function that is continuous at some x0 but not differentiable at x0.

is f(x) = |x| a right answer?
• Apr 22nd 2010, 05:35 PM
Drexel28
Quote:

Originally Posted by dgmath
I need an example of a function that is continuous at some x0 but not differentiable at x0.

is f(x) = |x| a right answer?

Yes. Or a uniformly continuous function which is nowhere differentiable. In fact, given a continuous function $\displaystyle f:I\to\mathbb{R}$ you may construct a continuous function $\displaystyle g:I\to\mathbb{R}$ such that $\displaystyle \|f-g\|_{\infty}<\varepsilon$ for some fixed but arbitrary $\displaystyle \varepsilon>0$ and which is nowhere differentiable. Your answer also shows that given a function $\displaystyle f$ which is differentiable at $\displaystyle x_0$ that the function $\displaystyle f(x)+|x-x_0|$ is not differentiable at $\displaystyle x=x_0$ giving and infinitely weaker form of the aforementioned theorem.