How to find the moments using the characteristic function?

In summary: You'll get two different answers.In summary, the Cauchy distribution has no moments because it does not have a Taylor expansion around the origin, as the characteristic function is not differentiable at t=0. This can be observed by looking at the graph of the function, which has a sharp turn at t=0. Calculating the derivative at t=0 for t<0 and t>0 will result in two different values, further supporting this conclusion.
  • #1
Neothilic
21
5
TL;DR Summary
How do you show the Cauchy distribution has no moments, but using the characteristic function?
I have the characteristic function of the Cauchy distribution ##C(t)= e^{-(\mid t \mid)}##. Now, how would I show that the Cauchy distribution has no moments using this? I think you have to show it has no Taylor expansion around the origin. I am not sure how to do this.
 
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  • #2
I can't say much about the first part of your post, but for there to be a Taylor series expansion the function has to be differentiable at the origin. It's not, just draw a graph and take a look at it.

To formally prove it, just calculate what the derivate is to the left of 0, and the right of 0, and observe they are not equal.
 
  • #3
Office_Shredder said:
I can't say much about the first part of your post, but for there to be a Taylor series expansion the function has to be differentiable at the origin. It's not, just draw a graph and take a look at it.

To formally prove it, just calculate what the derivate is to the left of 0, and the right of 0, and observe they are not equal.

I have forgotten what you explicitly mean by calculating the derivative is to the left, and to the right. Could you further elaborate? Surely as the function is of modulus t, this wouldn't apply?
 
  • #4
Have you tried drawing a plot of the function yet? There's a sharp corner at t=0. On the left of 0 it has one slope, on the right of 0 it has another slope. Similar to how the graph of |t| has a slope of -1 at 0 on the left, and 1 at 0 on the right.
 
  • #5
Office_Shredder said:
Have you tried drawing a plot of the function yet? There's a sharp corner at t=0. On the left of 0 it has one slope, on the right of 0 it has another slope. Similar to how the graph of |t| has a slope of -1 at 0 on the left, and 1 at 0 on the right.
I have looked at the plot of the function. What would that do to help? As there is a sharp turn at ##t=0##, surely that further enhances my conclusion?
 
  • #6
Yeah, I mean that's pretty much it. If you want to prove that the derivative doesn't exist, you can calculate what the derivative at 0 is when restricting to t<0 vs t>0
 

1. What is a characteristic function?

A characteristic function is a mathematical function that describes the probability distribution of a random variable. It is the Fourier transform of the probability density function (PDF) and provides information about the moments of the distribution.

2. How do you find the moments using the characteristic function?

To find the moments using the characteristic function, you can use the inverse Fourier transform. This will give you the PDF, from which you can calculate the moments using standard formulas.

3. What are the advantages of using the characteristic function to find moments?

The characteristic function allows for a more efficient and accurate way to find moments compared to traditional methods. It also works for both discrete and continuous distributions, making it a versatile tool for statistical analysis.

4. Are there any limitations to using the characteristic function to find moments?

One limitation is that the characteristic function may not exist for all distributions. In addition, the inverse Fourier transform can be complex and require advanced mathematical skills. It is also important to have a good understanding of the properties of characteristic functions to accurately interpret the results.

5. Can the characteristic function be used to find higher order moments?

Yes, the characteristic function can be used to find any order of moments. By taking the derivative of the characteristic function, you can find the moments of any order. This makes it a powerful tool for analyzing the properties of a distribution.

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