Does there exists a function f:R→R which is differentiable only at the point 0.?
My attempt : I found the answer here Is there a function f:R→R that has only one point differentiable?
But i didn't understands the answer , my doubts given below
Answer
Because while xp(x) is continuous at 0, it is not differentiable.
In particular, the fraction
(0+h)p(0+h)−0p(0)h
has value 0 or 1 depending on whether h is rational or not. So it has no limit as h→0, which by definition of derivative means that xp(x) had no derivative at 0.
On the other hand, the fraction
(0+h)2p(0+h)−02p(0)h
has value h or 0 depending on whether h is rational or irrational. Thus it does have a limit as h→0, which is to say that x2p(x) has derivative 0 at x=0.
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