Residue Question: Solving for Singularities and Integrals | Homework Help

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In summary: I don't understand why you have to use the positive residue. What piece of information tells you that the negative part isn't inside the unit circle.
  • #1
Applejacks
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Homework Statement



http://imageshack.us/photo/my-images/593/aaayl.png/

http://imageshack.us/photo/my-images/593/aaayl.png/

Homework Equations





The Attempt at a Solution



I already solved part b but I put it up there for part c. I know that there is a singularity at a=cost. However, maximum of cost=1 and a>1 so there is no singularity? Hence the integral equals zero. However, I don't get what the solution from part b tell us
 
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  • #2
The integral of 1*dt for t=0 to 2*pi isn't zero. And 1 doesn't have any singularities either. Because that's a REAL integral, it's not a complex integral. Why don't you try to express it as an integral dz instead of dt where z=e^(it) before you decide it's zero? You should find it has a lot to do with part b.
 
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  • #3
[itex]\int\frac{dz}{iz (a-cos(-iln(z)))}[/itex] is what I get. Then using Cauchy's Integral Formula:

2[itex]\pi[/itex]i*f(0) should equal to the integral. However, f(0) is undefined because of the log.
 
  • #4
Applejacks said:
[itex]\int\frac{dz}{iz (a-cos(-iln(z)))}[/itex] is what I get. Then using Cauchy's Integral Formula:

2[itex]\pi[/itex]i*f(0) should equal to the integral. However, f(0) is undefined because of the log.

Use cos(t)=(exp(it)+exp(-it))/2=(z+1/z)/2. Use Euler's formula. Don't bring log's into it. That's big trouble.
 
  • #5
I see. I end up getting the same function in part b but with a -2/i in front.

So we have 2ipi*(sum of res)*-i/2= the integral
However, I'm getting zero for the sum of the residue.
[itex]\frac{1}{(z-(a+\sqrt{a^2-1}))(z-(a-\sqrt{a^2-1})}[/itex]

The residues are [itex]\frac{1}{2\sqrt{a^2-1}}[/itex] and-[itex]\frac{1}{2\sqrt{a^2-1}}[/itex]

Is this correct?
 
  • #6
Applejacks said:
I see. I end up getting the same function in part b but with a -2/i in front.

So we have 2ipi*(sum of res)*-i/2= the integral
However, I'm getting zero for the sum of the residue.
[itex]\frac{1}{(z-(a+\sqrt{a^2-1}))(z-(a-\sqrt{a^2-1})}[/itex]

The residues are [itex]\frac{1}{2\sqrt{a^2-1}}[/itex] and-[itex]\frac{1}{2\sqrt{a^2-1}}[/itex]

Is this correct?

It would be, except you only sum over the residues inside the unit circle. Only one is inside. Which one is it??
 
  • #7
Well I'm looking at the two singularities:

a+[itex]\sqrt{a^2-1}[/itex]
a-[itex]\sqrt{a^2-1}[/itex]

For a>1, the second one tends to zero. So we should have
2ipi*-i/2 *sum (res)=pi* [itex]\frac{-1}{2\sqrt{a^2-1}}[/itex]?
 
  • #8
Applejacks said:
Well I'm looking at the two singularities:

a+[itex]\sqrt{a^2-1}[/itex]
a-[itex]\sqrt{a^2-1}[/itex]

For a>1, the second one tends to zero. So we should have
2ipi*-i/2 *sum (res)=pi* [itex]\frac{-1}{2\sqrt{a^2-1}}[/itex]?

The real integral you started with certainly isn't negative. Check that again. I don't think you did it very carefully.
 
  • #9
Okay I found my mistake. It should have been
[itex]\frac{2z}{(z-(a+\sqrt{a^2-1}))(z-(a-\sqrt{a^2-1})}[/itex]

Now the residues are:
[itex]\frac{-a-\sqrt{a^2-1}}{\sqrt{a^2-1}}[/itex]
[itex]\frac{a-\sqrt{a^2-1}}{\sqrt{a^2-1}}[/itex]

So finally I have 2ipi*sum(res)=2ipi* -2=-4ipi

Is this right?
 
  • #10
Applejacks said:
Okay I found my mistake. It should have been
[itex]\frac{2z}{(z-(a+\sqrt{a^2-1}))(z-(a-\sqrt{a^2-1})}[/itex]

Now the residues are:
[itex]\frac{-a-\sqrt{a^2-1}}{\sqrt{a^2-1}}[/itex]
[itex]\frac{a-\sqrt{a^2-1}}{\sqrt{a^2-1}}[/itex]

So finally I have 2ipi*sum(res)=2ipi* -2=-4ipi

Is this right?

NO. i) Look at the integral you are trying to solve. It's positive and real, right? -4ipi is neither. And ii) part b gave you the residue you need and you said you had already solved that part. It looked like you had it right in post 5. Why don't you start this whole thing from the beginning and show all of your steps and I'm sure someone will point out where you made a mistake.
 
  • #11
Sorry ignore my last post. I forgot to transform dt into dz.

Here is my working out:
http://imageshack.us/f/215/unled2lwl.png/

I just don't understand why I have to use the positive residue. What piece of information tells me that the negative part isn't inside the unit circle.
 
  • #12
Applejacks said:
Sorry ignore my last post. I forgot to transform dt into dz.

Here is my working out:
http://imageshack.us/f/215/unled2lwl.png/

I just don't understand why I have to use the positive residue. What piece of information tells me that the negative part isn't inside the unit circle.

The poles of your function are at z=a+sqrt(a^2-1) and z=a-sqrt(a^2-1). Pick which one satisfies |z|<1. It isn't the first one, is it? Why not??
 
  • #13
Well that's sort of my point. The first one is outside the unit circle since a>1, hence z becomes >1. Only the second one is inside the circle so how come I'm not using that one instead?
 
  • #14
Applejacks said:
Sorry ignore my last post. I forgot to transform dt into dz.

Here is my working out:
http://imageshack.us/f/215/unled2lwl.png/

I just don't understand why I have to use the positive residue. What piece of information tells me that the negative part isn't inside the unit circle.

Applejacks said:
Well that's sort of my point. The first one is outside the unit circle since a>1, hence z becomes >1. Only the second one is inside the circle so how come I'm not using that one instead?

You are! Call the two poles r+ and r-. Evaluate the residue at r-. Your function is proportional to 1/((z-r+)(z-r-)). So the residue is lim z->r- (z-r-)*1/((z-r+)(z-r-))=lim z->r- 1/(z-r+)=1/((r-)-(r+)). That's the one one that gives you the negative residue. Work it out carefully!
 
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  • #15
I see now. However, my final answer should have 2pi instead of pi/2 right? I reworked it and that's what I got.
 
  • #16
Applejacks said:
I see now. However, my final answer should have 2pi instead of pi/2 right? I reworked it and that's what I got.

Yes, it should be 2*pi/sqrt(a^2-1).
 

1. What is residue question and why is it important?

Residue question is a mathematical problem that involves solving for singularities and integrals. It is important because it allows us to find the values of complex functions at specific points, which can be useful in various fields such as physics, engineering, and finance.

2. How do you determine the residue of a function?

To determine the residue of a function, we first need to find the singularities of the function. Then, we use the formula Res(f, z0) = lim(z->z0) [(z-z0)*f(z)] to calculate the residue at each singularity. The sum of all residues at all singularities will give us the total residue of the function.

3. What is the difference between a simple pole and a higher-order pole?

A simple pole is a singularity of a function where the limit of the function as it approaches the pole is finite. On the other hand, a higher-order pole is a singularity where the limit of the function as it approaches the pole is infinite. In other words, a simple pole has a first-order singularity while a higher-order pole has a singularity of order greater than one.

4. How do you integrate a function with a singularity using the residue theorem?

To integrate a function with a singularity using the residue theorem, we first need to find the residues of the function at all singularities. Then, we use the formula ∮f(z)dz = 2πi * (sum of residues) to calculate the integral. This method is often used when the traditional integration methods are not applicable due to the presence of singularities.

5. Can the residue theorem be applied to any function?

The residue theorem can only be applied to functions that are analytic in a region and have isolated singularities. In other words, the function should be continuous and differentiable in the region except at a finite number of points. If these conditions are met, then the residue theorem can be applied to the function.

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