What it means to be infinitely differentiable

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SUMMARY

The discussion clarifies the distinction between "infinitely many times continuously differentiable" and "infinitely many times differentiable." The term C^∞ specifically refers to functions that are infinitely differentiable, meaning all derivatives exist and are continuous at a given point. It is established that while a function can be infinitely differentiable, the operation of differentiating infinitely many times is not well-defined. The continuity of the (n-1)th derivative is essential for the existence of the nth derivative.

PREREQUISITES
  • Understanding of calculus concepts, particularly derivatives
  • Familiarity with the definitions of continuous functions
  • Knowledge of the notation C^∞ in mathematical analysis
  • Basic comprehension of limits and continuity in real analysis
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  • Study the properties of C^∞ functions in mathematical analysis
  • Learn about the implications of differentiability in real-valued functions
  • Explore the concept of continuity and its relationship with differentiability
  • Investigate the definitions and examples of functions that are not infinitely differentiable
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Mathematics students, educators, and anyone interested in advanced calculus and real analysis concepts, particularly those focusing on differentiability and continuity in functions.

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<Moderator's note: Moved from a homework forum.>

1. Homework Statement

I am wondering if it is the same to say :

Function f is infinitely many times continuously differentiable
AND
Function f is infinitely many times differentiable

And if it is not the same, which one defines: C^infinity ?

Homework Equations

The Attempt at a Solution

 
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Infinitely differential C^{\infty} refers to a property at a particular point. However to get the nth derivative at that point, the (n-1)th derivative needs to be continuous at that point.
 
It's worth mentioning that to differentiate a function "infinitely many times" is an undefined operation. An "Infinitely differentiable" function is one whose Nth derivative exits for any finite integer N > 0.
 
A differentiable function is continuous.
 

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