# diferntialbility of a function

• Oct 18th 2010, 08:29 PM
Jskid
diferntialbility of a function
If a function is differentiable, does that mean its higher order derivatives are also differentiable?
• Oct 18th 2010, 08:41 PM
Jskid
I guess not since a polynomial's derivative will (eventually) have a removable discontinuity at x=0
• Oct 19th 2010, 03:53 AM
HallsofIvy
Oh, that's not true! A polynomial is infinitely differentiable. I can not imagine why you would think it will "eventually have a removable discontinuity at x= 0". The derivative of any polynomial is eventually identically 0 and that's a very differentiable function!

Instead, to find an example of a differentiable function that is not infinitely differentiable, start with a discontinuous function and integrate it.

For example, integrating f(x)= -1 if x< 0 and 1 if $x\ge 0$, which is not continuous at x=0, gives g(x)= -x if x< 0 and g(x)= x if $x\ge 0$ (I have chosen the constant of integration to be 0). In other words, g(x)= |x| which is continuous for all x but not differentiable at x= 0. Integrating again gives $h(x)= -x^2/2$ if x< 0 and $h(x)= x^2/2$ if $x\ge 0$. That function is differentiable at x= 0 but not twice differentiable. Integrating again would give a function that is twice differentiable but not three times differentiable.
• Oct 19th 2010, 09:12 AM
Jskid
That answers my initial question but I have trouble with something else.
Let $f(x)=x^2$ then $f'(x)= 2x$
In general $\frac{dx}{xy}=nx^{n-1}$ but at x=0 for n=1 the function is undefined.
So how would $f''(x)=0$?
• Oct 21st 2010, 10:32 AM
Jskid
Am I right? Since $\frac{dy}{dx} 2x = 2(1)x^{1-1}$ but at x=0 $2(0)^0$ is undefined.