# Examples where it's Riemann integrable but no derivative exists at pts

1. Nov 17, 2008

### Nusc

What is an example where it's Riemann integrable int(f(t),t,a,x) but no derivative exists at certain pts?

2. Nov 17, 2008

### HallsofIvy

The function f(x)= 0 if $x\le 0$, 1 if x> 0 is integrable but has no derivative at x= 0. More generally, if f(x) has finite "jump" discontinuities at some points, it is still Riemann integrable but is not differentiable at those points.

3. Nov 18, 2008

or $$f(x)=\left| x\right|$$

4. Nov 18, 2008

### HallsofIvy

Those two examples also have the property that while $F(x)= \int f(t)dt$ is defined, F(x) itself has no dervative at x= 0.

5. Nov 19, 2008

### mathwonk

there are functions which are continuous everywhere hence integrable, but differentiable nowhere. perhaps the famous dirichlet function which equals zero at irrationals and 1/q at p/q is even differentiable nowhere. since it is continuous a.e. it is integrable.

6. Nov 19, 2008

$$V_{3}[\tex] 7. Nov 20, 2008 ### arildno An example of a continuous (and hence integrable) function that is nowhere differentiable is the Weierstrass function: [tex]F(x)=\sum_{n=0}^{\infty}\frac{\sin((n!)^{2}x)}{n!}$$

8. Nov 20, 2008

### Doodle Bob

I'm not sure about this. Isn't $F(x)= \int_0^x |t|dt$ differentiable at 0? It is the piecewise function given by F(x)=x^2 for x>0 and F(x)=-x^2 for x>0 and F(0)=0.

9. Nov 20, 2008

### HallsofIvy

Yes, you are right. That was an error on my part.