Monday, February 8, 2016

real analysis - why here x2 is used ? why not x?




Does there exists a function f:RR which is differentiable only at the point 0.?




My attempt : I found the answer here Is there a function f:RR that has only one point differentiable?




But i didn't understands the answer , my doubts given below



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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 h0, 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 h0, which is to say that x2p(x) has derivative 0 at x=0.


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