Integrals that can't be evaluated

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Discussion Overview

The discussion revolves around the integral of the function (1+(1/x))^x, specifically why it cannot be expressed in terms of elementary functions. Participants explore the nature of the function, its integrability, and the implications of evaluating integrals in closed form.

Discussion Character

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

Main Points Raised

  • One participant questions why the integral of (1+(1/x))^x cannot be evaluated, noting its limit approaches e as x approaches infinity.
  • Another participant clarifies that the function is Riemann integrable on intervals not including x=0, suggesting the inquiry is about indefinite integrability in terms of elementary functions.
  • A participant introduces the indefinite integral F(x) = ∫(1 + (1/t))^t dt, stating that while it is an indefinite integral, no known combination of elementary functions represents it.
  • Some participants express frustration over the lack of a definitive answer, with one mentioning that Mathematica cannot integrate the function, implying it may not be expressible in elementary terms.
  • There is a discussion about the meaning of "can't be evaluated," with emphasis on the distinction between evaluating a function and evaluating it in terms of elementary functions.
  • One participant argues that many integrable functions cannot be expressed in terms of known functions, suggesting the issue lies more in the limitations of known functions than in the function itself.

Areas of Agreement / Disagreement

Participants generally agree that the function is Riemann integrable on certain domains, but there is no consensus on whether it can be expressed in terms of elementary functions. The discussion remains unresolved regarding the nature of its indefinite integral.

Contextual Notes

Participants highlight that the function is continuous and bounded except at x=0, and there are unresolved questions about the definitions and implications of integrability and evaluation in terms of elementary functions.

lvlastermind
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This is an inquiry that has been an itch I can't reach.


Why can't the integral (1+(1/x))^x be evaluated?

I know the limit {n->inf} (1+(1/x))^x = e but I don't understand why the function doesn't have an integral.

Thanks in advance
 
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The function is continuous and bounded everywhere but on intervals including x=0, so on any domain that does not include 0, the function is Riemann-integrable. Did you mean indefinitely integrable as a closed form expression involving a finite combination of elementary functions?
 
slider142 said:
The function is continuous and bounded everywhere but on intervals including x=0, so on any domain that does not include 0, the function is Riemann-integrable. Did you mean indefinitely integrable as a closed form expression involving a finite combination of elementary functions?



Yes, I meant indefinitely...
 
Since it is Riemann integrable on every interval not containing 0, let
F(x) = \int_0^x (1 + \frac{1}{t})^t dt
Then F is an indefinite integral of your expression. Unfortunately, I do not know of any combination of elementary functions that is equivalent to F. Proving that there is no such finite combination is not a trivial matter. Have a look at http://www.soton.ac.uk/~adf/pubstore/pub20.pdf .
 
Last edited by a moderator:
slider142 said:
Since it is Riemann integrable on every interval not containing 0, let
F(x) = \int_0^x (1 + \frac{1}{t})^t dt
Then F is an indefinite integral of your expression. Unfortunately, I do not know of any combination of elementary functions that is equivalent to F.


Agreed... Hence I'm back to square one...

Anyone know?
 
Sorry to bump this thread but I'm really interested about the answer...
 
The problem is "what do you mean by "can't be evaluated"? The function "1+ (1/x)x" is continuous and so certainly can be evaluated. Do you mean "evaluated in terms of elementary functions"? That's a completely different question!
 
HallsofIvy said:
The problem is "what do you mean by "can't be evaluated"? The function "1+ (1/x)x" is continuous and so certainly can be evaluated. Do you mean "evaluated in terms of elementary functions"? That's a completely different question!

Yes, indefinite integral is what I'm looking for...
 
  • #10
This is the last time I'll bump this thread and if no one has an answer I'll let it die...
 
  • #11
Good. Your question really doesn't make much sense to begin with. "Almost all" integrable functions cannot be integrated in terms of functions we know simply because the set of functions we know is so small. It's not a matter of "why can we not integrate this function in terms of functions we know" but of "why can we integrate certain functions".
 

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