Evaluation of a definite integral

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Discussion Overview

The discussion revolves around the evaluation of a definite integral involving the function exp(-β²r²)r^(n-1) from 0 to ∞. Participants are exploring the implications of substituting the limits of integration, particularly how the function behaves at these limits and the resulting value of the integral.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express confusion about how the definite integral evaluates to zero, particularly at the limits of 0 and ∞.
  • There is a discussion about the correct formulation of the integral, with some participants suggesting it should be written as $$\int_0^\infty e^{-\beta^2r^2}r^{n-1}\;dr$$.
  • One participant notes that when substituting r=0, the expression involves evaluating 0 raised to the power of (n-1), leading to uncertainty about its value.
  • Another participant points out that if n=1, the expression would lead to 0^0, which is considered an indeterminate form, raising further questions about the evaluation in that case.
  • There is mention of a factor of 1/(n-1) in the integral, which adds complexity to the evaluation.
  • Participants discuss the misconception that zero raised to any power is 1, with one participant acknowledging the confusion and seeking clarification on the rules of exponentiation.

Areas of Agreement / Disagreement

Participants generally agree on the formulation of the integral but disagree on the implications of evaluating the limits, particularly at r=0 and the treatment of 0^0. The discussion remains unresolved regarding the evaluation of the integral under these conditions.

Contextual Notes

There are limitations regarding the assumptions about the values of n and β, as well as the implications of evaluating expressions involving zero raised to various powers. The discussion also highlights the need for clarity on the definitions and rules of exponentiation.

dyn
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I am looking at a solution to an question and I don't understand how the value of the following definite integral comes out to be zero ? The following function is evaluated from 0 to ∞ with r being the variable

## exp(-β^2r^2)r^nr^-1/(n-1)## That should read r raised to the power of (n-1)
I presume when the value of ∞ is input the negative exponential turns out to be zero but I don't understand how inputting zero gives a value of zero ?
 
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dyn said:
I am looking at a solution to an question and I don't understand how the value of the following definite integral comes out to be zero ? The following function is evaluated from 0 to ∞ with r being the variable

## exp(-β^2r^2)r^nr^-1/(n-1)## That should read r raised to the power of (n-1)
which? where?
Did you mean to write: $$\int_0^\infty e^{-\beta^2r^2}r^{n-1}\;dr$$

I presume when the value of ∞ is input the negative exponential turns out to be zero but I don't understand how inputting zero gives a value of zero ?

What do you think it should come out to?
What is beta? What is "n"?
 
Simon Bridge said:
which? where?
Did you mean to write: $$\int_0^\infty e^{-\beta^2r^2}r^{n-1}\;dr$$



What do you think it should come out to?
What is beta? What is "n"?

You have got the function right but it also has a factor of 1/(n-1). This function was derived from integration by parts. It just needs evaluating with the values ∞ and 0. When I input 0 I have exp(0) which is 1 and 0 to the power (n-1) which I thought is also 1. But overall the answer should be zero
 
Oh you mean you've done an integral and you have ended up at this stage: $$\frac{1}{n-1}\left[ e^{-\beta^2r^2} r^{n-1}\right]_0^\infty$$
When I input 0 I have exp(0) which is 1 and 0 to the power (n-1) which I thought is also 1. But overall the answer should be zero
... when you put ##r=0## into ##r^{(n-1)}## you get ##0^{n-1}## right?
What is zero multiplied by itself? i.e. if n=3, then n-1=2 and ##0^2=0\times 0 = ?##
 
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Thanks for that. Somehow I was under the impression that zero raised to any power was 1 !
 
You just got a rule stuck in your head, that's all.
Happens to everyone ;)
 
Simon Bridge said:
You just got a rule stuck in your head, that's all.
Happens to everyone ;)

Thanks. I did get the wrong rule stuck in my head. But what would happen at r=o if n=1 then I would have zero to the power of zero which I believe is 1 ? I also have a zero on the denominator. Is it impossible to evaluate in this case ? Incidentally the question did state n≥2 so I am just asking for future reference.
 
You should review the rules of exponentiation:

http://en.wikipedia.org/wiki/Exponentiation

The expression 0^0 (zero raised to the zeroth power) is considered, in general, an indeterminate form. 0^n is defined only for integers n > 0. There may be certain formulas which require that 0^0 be evaluated as equal to one, but these are exceptions.
 

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