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Limit for a problem of convergence

  • Thread starter gentsagree
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  • #1
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Homework Statement



I ultimately want to discuss convergence of the integral

[tex]\int_{0}^{\infty}\frac{1}{\sqrt{x}e^{\sqrt{x}}}dx[/tex][/B]


Homework Equations



[tex]\int_{c}^{\infty}\frac{dx}{x^{p}}[/tex]

is convergent near x approaching infinity for p>1

3. The Attempt at a Solution


While I understand that the integral converges near 0, as x->infinity I find, for the integrand

[tex]\lim_{x \rightarrow \infty }\frac{1}{\sqrt{x}e^{\sqrt{x}}}=\lim_{x \rightarrow \infty }\frac{1}{\sqrt{x}\left( 1+\sqrt{x} + \frac{x}{2} + \cdots \right) }=\lim_{x \rightarrow \infty }\frac{1}{\left( \sqrt{x} + x + \frac{\sqrt{x}x}{2} + \cdots \right) }[/tex]

but the solution given to me says that I should end up with

[tex]\frac{1}{\left(\frac{\sqrt{x}x}{2} + \cdots \right) }[/tex]

which converges as p would be always greater than 3/2, but I don't see how the limit gets rid of the first two terms in the denominator.

Thanks!
 

Answers and Replies

  • #2
Ray Vickson
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Homework Statement



I ultimately want to discuss convergence of the integral

[tex]\int_{0}^{\infty}\frac{1}{\sqrt{x}e^{\sqrt{x}}}dx[/tex][/B]


Homework Equations



[tex]\int_{c}^{\infty}\frac{dx}{x^{p}}[/tex]

is convergent near x approaching infinity for p>1

3. The Attempt at a Solution


While I understand that the integral converges near 0, as x->infinity I find, for the integrand

[tex]\lim_{x \rightarrow \infty }\frac{1}{\sqrt{x}e^{\sqrt{x}}}=\lim_{x \rightarrow \infty }\frac{1}{\sqrt{x}\left( 1+\sqrt{x} + \frac{x}{2} + \cdots \right) }=\lim_{x \rightarrow \infty }\frac{1}{\left( \sqrt{x} + x + \frac{\sqrt{x}x}{2} + \cdots \right) }[/tex]

but the solution given to me says that I should end up with

[tex]\frac{1}{\left(\frac{\sqrt{x}x}{2} + \cdots \right) }[/tex]

which converges as p would be always greater than 3/2, but I don't see how the limit gets rid of the first two terms in the denominator.

Thanks!
Much easier: change variables. There is a particularly "obvious" transformation, but if I tell you exactly what it is I will be violating PF rules.
 
  • #3
Zondrina
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The integral indeed converges because you know the integral below on the right converges:

$$\int_0^{\infty} \frac{1}{\sqrt{x} e^{\sqrt{x}}} \space dx \leq \displaystyle \lim_{a \rightarrow \infty} \int_0^a \frac{1}{e^{\sqrt{x}}} \space dx$$

So to determine what the integral on the left converges to, note that:

$$\sqrt{x} e^{\sqrt{x}} = \sum_{n}^{\infty} \frac{x^{\frac{n+1}{2}}}{n!}$$

So that:

$$\int_0^{\infty} \frac{1}{\sqrt{x} e^{\sqrt{x}}} dx = \int_0^{\infty} \frac{1}{\sum_{n}^{\infty}\frac{x^{\frac{n+1}{2}}}{n!}} dx$$
 
  • #4
haruspex
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The integral indeed converges because you know the integral below on the right converges:

$$\int_0^{\infty} \frac{1}{\sqrt{x} e^{\sqrt{x}}} \space dx \leq \displaystyle \lim_{a \rightarrow \infty} \int_0^a \frac{1}{e^{\sqrt{x}}} \space dx$$
I don't see how you get that inequality. Is that what you meant?
Anyway, Ray's method is much simpler.
 
  • #5
LCKurtz
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The integral indeed converges because you know the integral below on the right converges:

$$\int_0^{\infty} \frac{1}{\sqrt{x} e^{\sqrt{x}}} \space dx \leq \displaystyle \lim_{a \rightarrow \infty} \int_0^a \frac{1}{e^{\sqrt{x}}} \space dx$$


The integral is improper at ##0## and your inquality fails near ##0##.
 
  • #6
Zondrina
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Yes I don't know what I was doing using series, I wasn't fully awake when I wrote that, sorry. There is a much simpler way that has been mentioned already.
 
  • #7
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There are two discontinuities here: x = 0 and x = infinity

so rewrite as lim a→0 (lim b→∞ (∫dx/(√x e*√x) from a to b))
u = √x
2du = dx/√x
lim a→0 (lim b→∞ (∫e^-u du from a to b))
= lim a→0 (lim b→∞ -e^-b + e^-a)
= 0 +1
= 1
 
  • #8
haruspex
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There are two discontinuities here: x = 0 and x = infinity
I don't think it means anything to say there's a discontinuity at infinity. The definition of an integral to infinity is the limit as the range tends to infinity.
2du = dx/√x
You later lost the 2.
 
  • #9
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I don't think it means anything to say there's a discontinuity at infinity. The definition of an integral to infinity is the limit as the range tends to infinity.

You later lost the 2.
Yes you are right, it does not make sense to say that there's a discontinuity at infinity. I just meant that we can not treat as a normal limit of integration and we are gonna have to take the limit as that limit goes to infinity. Thank you from pointing that out.

And yup, i must gave forgotten to write the two. so the correct answer is 2, not 1. Thanks again.
 
  • #10

but the solution given to me says that I should end up with

[tex]\frac{1}{\left(\frac{\sqrt{x}x}{2} + \cdots \right) }[/tex]
The solution given to you says you should get something with x in it after taking limits?

If you can find a value for the integral, then you know it converges. Taking the limit of the integrand as x goes to infinity will only tell you whether or not the integrand goes to zero. In this case, it does - however you still don't know if it converges or diverges. You can recognize that the integrand is just the derivative of the function [itex]-2e^{-\sqrt{x}}[/itex] and so you can use the fundamental theorem of calculus (part 2) and then evaluate the limits.
 

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