When is it OK to pass the limit to the exponent

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    Exponent Limit
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Discussion Overview

The discussion revolves around the conditions under which it is permissible to pass the limit to the exponent in expressions involving limits and exponential functions. The scope includes mathematical reasoning and the properties of continuous functions.

Discussion Character

  • Mathematical reasoning

Main Points Raised

  • One participant questions the justification for passing the limit to the exponent in expressions like \(\lim_{n\to\infty} e^{\ln x^{1/n}} = e^{\lim_{n\to\infty} \ln x^{1/n}}\) and \(\lim_{n\to\infty} 2^{f(n)} = 2^{\lim_{n\to\infty} f(n)}\).
  • Another participant states that the limit of a composition of functions can be taken if the outer function is continuous at the limit of the inner function, asserting that exponential functions are continuous everywhere, thus allowing the limit to be passed in both cases.
  • A further contribution reiterates the definition of continuity, indicating that if \(f\) is continuous at a point, then \(\lim_{x\to a} f(g(x)) = f(\lim_{x\to a} g(x))\) holds true.
  • One participant emphasizes that exponential functions are continuous everywhere, supporting the earlier claims about the validity of passing limits in the discussed scenarios.

Areas of Agreement / Disagreement

Participants generally agree on the continuity of exponential functions and the conditions under which limits can be passed to the exponent. However, the initial question about justification remains open to further exploration.

Contextual Notes

The discussion does not address specific cases where the continuity of functions may be in question or where limits might not behave as expected, leaving some assumptions unexamined.

clueless...
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when is it justified to do something like this

[tex]\lim_{n\to\infty} e^{lnx^{1/n}} = e^{\displaystyle\lim_{n\to\infty}lnx^{1/n}}[/tex]

or something like this

[tex]\lim_{n\to\infty} 2^{f(n)} = 2^{\displaystyle\lim_{n\to\infty}f(n)}}[/tex] ?

I am assuming that I can do something like this in both cases, but why?

thank you.
 
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in general
lim f(g(n))=f(lim g(n))
holds when f is continuous at lim g(n)
exponential functions are everywhere continuous so this can be done in both cases
 
In fact [itex]\lim_{x\to a}f(x)= f(\lim_{x\to a} x)= f(a)[/itex], from which [itex]\lim_{x\to a}f(g(x))= f(\lim_{x\to a}g(x))[/itex]
is the definition of "f is continuous at x= a".
 
Wherever it is continuous which is everywhere
 

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