Jordan's Lemma technicalities - where does sin(mz) go

  • Thread starter thomas49th
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In summary, the conversation is about understanding why the value of m is important in the given integral and how the sin(mz) term disappears in the integration process. The writer of the lecture notes uses the complex form of the integral to show how the complex part, which contains the sin(mz) term, becomes zero due to the fact that the integrand is an odd function. The conversation also touches on the role of the variables x and z in the integration process.
  • #1
thomas49th
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Homework Statement



I don't understand why it's so great m>0 here

http://gyazo.com/cfa21b39653275a38ec80722b7faa58b
From these lectures notes (I think they are generally pretty good)

Where has the sin(mz) gone?

When going from 4.67 to 4.68 has

e^imz = cos(mz) + isin(mz) been used? I'm guessing that was the trick, but I don't see where sin(mz) has pootled off to?
 
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  • #2
thomas49th said:

Homework Statement



I don't understand why it's so great m>0 here

http://gyazo.com/cfa21b39653275a38ec80722b7faa58b
From these lectures notes (I think they are generally pretty good)

Where has the sin(mz) gone?

When going from 4.67 to 4.68 has

e^imz = cos(mz) + isin(mz) been used?
Yes.
thomas49th said:
I'm guessing that was the trick, but I don't see where sin(mz) has pootled off to?

Instead of looking at ##\int \frac{cos(mx)dx}{a^2 + x^2}##, the writer of the notes is looking at ##\int \frac{e^{mz}dz}{a^2 + z^2}## and getting a value. He also notes that the complex part, which contains the sin(mx) numerator, turns out to be zero, due to the fact that the integrand is an odd function. The integral of an odd function over (-∞, ∞) is zero.
 
  • #3
thomas49th said:

Homework Statement



I don't understand why it's so great m>0 here

http://gyazo.com/cfa21b39653275a38ec80722b7faa58b
From these lectures notes (I think they are generally pretty good)

Where has the sin(mz) gone?

When going from 4.67 to 4.68 has

e^imz = cos(mz) + isin(mz) been used? I'm guessing that was the trick, but I don't see where sin(mz) has pootled off to?

At the bottom he says that the sin(mx) integral vanishes, leaving only the cos(mx) part. It says that right in the document!

RGV
 
  • #4
I think I'm happy with the sin(mx) disappearing. The m variable is causing me grief now:

So for the variable m, what decides it. If I'm given a functions like

1/(x+1)^4

is the variable m in the e^imz equal to the overall degree of the function

ie would m=2 if

x^2/(x+1)^4 was my function?

Thanks
Thomas
 
  • #5
Neither m nor a is a variable - both are constants as far as the integration is concerned. The only variables are x and z.
 

FAQ: Jordan's Lemma technicalities - where does sin(mz) go

1. What is Jordan's Lemma and how does it relate to sin(mz)?

Jordan's Lemma is a theorem in complex analysis that describes the behavior of integrals with complex valued functions. It states that if a function approaches zero as a variable approaches infinity, then the integral of the function will also approach zero. In the case of sin(mz), the function approaches zero as z approaches infinity, thus the integral of sin(mz) will also approach zero.

2. What are the technicalities associated with Jordan's Lemma and sin(mz)?

One of the main technicalities associated with Jordan's Lemma and sin(mz) is the need for the function to be continuous and differentiable in the region of integration. If the function is not continuous or differentiable, then Jordan's Lemma cannot be applied. Additionally, the function must also approach zero as the variable approaches infinity in order for the theorem to be valid.

3. Are there any limitations to Jordan's Lemma when applied to sin(mz)?

Yes, there are limitations to Jordan's Lemma when applied to sin(mz). One limitation is that the theorem only applies to integrals with real valued functions. This means that the imaginary component of sin(mz) must be zero in order for Jordan's Lemma to be applicable. Another limitation is that the function must also be continuous and differentiable in the region of integration, as mentioned previously.

4. Can Jordan's Lemma be applied to other trigonometric functions?

Yes, Jordan's Lemma can be applied to other trigonometric functions, such as cos(mz) or tan(mz). As long as the function approaches zero as the variable approaches infinity and is continuous and differentiable in the region of integration, the theorem can be applied. However, the function must also have a real value, as Jordan's Lemma only applies to integrals with real valued functions.

5. What are the implications of Jordan's Lemma and sin(mz) in complex analysis?

Jordan's Lemma and sin(mz) have important implications in complex analysis. They allow for the evaluation of complex integrals by transforming them into real integrals, making them easier to solve. They also provide a way to evaluate integrals that would otherwise be difficult or impossible to solve using other methods. Overall, Jordan's Lemma and sin(mz) are powerful tools in the field of complex analysis.

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