Proof for {lim of exp = exp of lim}

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Discussion Overview

The discussion revolves around the proof of the limit property involving the exponential function, specifically the statement that if \( f(x) \) is "nice," then \( \lim_{x\rightarrow c} e^{f(x)}=e^{\lim_{x\rightarrow c} f(x)} \). Participants explore various approaches to understanding and proving this limit property, touching on concepts of continuity, power series, and the nature of the function \( f(x) \).

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses difficulty in finding appropriate resources for the proof of the limit property involving the exponential function.
  • Another suggests expanding the exponential function into a power series and applying the distributive property of limits.
  • A participant points out that using power series for proof may be circular, as continuity of the exponential function must be established first.
  • Some participants discuss the interpretation of "nice" functions, with one suggesting it implies having all orders of derivatives, while another questions whether smoothness is necessary, proposing that pointwise convergence might suffice.
  • One participant references the rule of composition of limits, suggesting that if \( f \) has a finite limit at \( c \), then the limit property follows from the continuity of the exponential function.
  • Another participant notes that continuity implies sequential continuity, although the reverse is not necessarily true.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement regarding the assumptions needed for the proof. While some agree on the importance of continuity, others question whether smoothness is a strict requirement, indicating that multiple competing views remain on the necessary conditions for the limit property.

Contextual Notes

There are unresolved assumptions regarding the nature of the function \( f(x) \) and the implications of continuity versus smoothness in the context of the limit property being discussed.

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I don't know how to Google appropriately for this, since the kind of keywords I use present me with search results that try to define the exponential function using limits instead of what I am trying to ask:

What does the proof look like for the following (assuming f(x) is "nice"). Any sites that can show this? Thanks.

$$\lim_{x\rightarrow c} e^{f(x)}=e^{\displaystyle\lim_{x\rightarrow c} f(x)}$$
 
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Consider expanding the exponential into power series then use the distributive property of limit.
 
blue_leaf77 said:
Consider expanding the exponential into power series then use the distributive property of limit.

That is a circular proof. In order to Taylor/MacLaurin expand a function, you need to have proven its continuity.
 
Hm. I wonder...

Define a function Φ(x) as Φ(x)=0 for x≤0; [itex]\phi(x)=e^{-\frac{1}{x^{2}}}[/itex] for x>0. Now Φ(x) is "nice" as it is C, even at x=0.
 
blue_leaf77 said:
Well I interpret the OP's wording of "assuming f(x) is "nice"" as to be having all orders of derivative.
Maybe, but is smoothness of ##f## necessary? Without considering details it looks like pointwise convergence at ##c## should be enough.
 
It seems that it is just an application of the rule of composition of limits (if ##f## nice means that it has a finite limit in ##c##).

The rule states that if ##h(x) \to b ## as ## x\to a##, and if ##g(x) \to \ell ## as ##x \to b##, then ##g(h(x)) \to \ell ## as ##x\to a##.
If you add ##g## continuous (like ##x\to\exp(x)##) then ##\ell = g(b)##
 
Last edited:
I think , like Fresh_42 said, that continuity is one of the simplest ways of going about it, continuity implies sequential continuity (though the converse is not true)
 

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