Tuesday, December 26, 2017

real analysis - Can it be that f and g are everywhere continuous but nowhere differentiable but that fcircg is differentiable?

So, I was just asking myself can something like this happen? I was thinking about some everywhere continuous but nowhere differentiable functions f and g and the natural question arose on can the composition fg be differentiable, in other words, can the operation of composition somehow "smoothen" the irregularities of f and g which make them non-differentiable in such a way that composition becomes differentiable?



So here is the question again:




Suppose that f and g are everywhere continuous but nowhere differentiable functions. Can fg be differentiable?





If such an example exists it would be interesting because the rule (f(g(x))=f(g(x))g(x) would not hold, and not only that it would not hold, it would not make any sense because f and g are not differentiable.

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