Does the Existence of a Sequence Imply the Existence of a Derivative?

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SUMMARY

The discussion centers on the mathematical proof that if a function f: R->R is differentiable at a point c, then the derivative f'(c) can be expressed as the limit of the difference quotient as n approaches infinity, specifically f'(c) = lim(n→∞)(f(c + 1/n) - f(c)). However, an example using the absolute value function |x| demonstrates that the existence of this limit does not guarantee the existence of the derivative at that point, highlighting a critical distinction in calculus.

PREREQUISITES
  • Understanding of limits in calculus
  • Familiarity with the definition of a derivative
  • Knowledge of differentiable functions
  • Concept of sequential limits
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  • Learn about the sequential criterion for limits
  • Investigate examples of functions that are continuous but not differentiable, such as |x|
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kbfrob

Homework Statement


If f: R->R is differentiable at c in R, show that:
f'(c)=lim(n{f(c+1/n)-f(c)})

Show by example that the existence of this sequence doesn't imply the existence of f'(c)

The Attempt at a Solution


I got the second part, with my example just being |x|, but I'm not sure about the first part.
 
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I assume the limit is to be taken as n -> infinity. Do you know the standard definition of the derivative of f at c? Can you take the definition in your problem and somehow change it to match the standard definition?
 


Show the work you've done, and think about the sequential criterion for limits.
 

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