How Can Simple Functions Help Find Lower Bounds for Integrals?

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Discussion Overview

The discussion centers on finding positive valued simple functions that can serve as lower bounds for two specific integrals. Participants explore potential functions and methods for approximating these integrals without using complex components or infinite series.

Discussion Character

  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • One participant requests simple functions that are less than or equal to two given integrals.
  • The integrals discussed are: 1) \(\int_{x}^{\infty}\frac {e^{-y}}{y}dy\) and 2) \(\int_{x}^{\infty} y e^{-y}dy\).
  • Another participant notes that the second integral can be integrated exactly to \(e^{-x}(1+x)\).
  • For the first integral, a suggestion is made to replace \(y\) in the denominator with \(e^y\) to simplify the integral, although this approach is described as yielding a poor lower bound.
  • Further exploration includes substituting \(y\) with \(e^{y/2}\) or \(e^{y-1}\) in the first integral, with a request for better ideas regarding this approach.
  • There is a clarification regarding the misunderstanding of which problem was being referred to, indicating some confusion among participants.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of the proposed methods for finding lower bounds, with no consensus reached on the best approach for the first integral.

Contextual Notes

Some assumptions about the nature of "simple functions" and the criteria for lower bounds remain unspecified, and the discussion includes unresolved mathematical steps related to the integrals.

ssd
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Looking for some positive valued simple functions which are less than (or equal to) the following two integrals (given in the following post).By simple I mean that they may not involve integrals or imaginary components or some infinite series. Again, the functions may not be as simple as f(x) =0.

Please find the integrals in the following post, as I could not fix the latex problem in this post.

Thanks for any idea.
 
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The integrals as referred in the previous post are as follows:


1/ \int_{x}^{\infty}\frac {e^{-y}}{y}dy , x>0

2/ \int_{x}^{\infty} y e^{-y}dy , x>0
 
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The second one can be integrated to (exactly) e^{-x}(1+x).

In the first one you can replace the y in the denominator by e^y which will give you an easy integral. It will be a pretty bad lower bound though.
 
Eighty said:
The second one can be integrated to (exactly) e^{-x}(1+x).

In the first one you can replace the y in the denominator by e^y which will give you an easy integral. It will be a pretty bad lower bound though.

Thank you very much, I missed the substitution in that.
EDIT: I also missed that it simply can also be done 'by parts'.

What I thought was to replace y in the denominator by e^y/2 or e^{y-1} in the other problem.
Any better idea about the second?...EDIT: I mean the other, problem no.1.
 
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Better how? It's an exact antiderivative. What do you want?

edit: You can edit your posts, you know. :) Click the EDIT button next to the QUOTE button.
 
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Eighty said:
Better how? It's an exact antiderivative. What do you want?

edit: You can edit your posts, you know. :)
Sorry for the misunderstanding, by 'second' I meant the other problem.
Thanks again for the help though.
 
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