Why Is the Characteristic Function of a Random Variable Complex-Valued?

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Discussion Overview

The discussion centers around the nature and significance of the characteristic function of a random variable, particularly its complex-valued nature. Participants explore its derivation, applications, and comparison with moment-generating functions, addressing both theoretical and practical aspects.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant notes that the characteristic function is a complex-valued function and questions whether this is the only reason for its use, seeking clarification on its derivation.
  • Another participant emphasizes that definitions are not derived and mentions that the characteristic function is useful for deriving the distribution function of a sum of independent random variables.
  • A different participant introduces moment-generating functions, stating that while they can uniquely identify a distribution under strict conditions, not all distributions possess them, unlike characteristic functions which exist for every distribution.
  • A participant shares a resource that they found helpful for understanding the derivation and applications of the characteristic function.

Areas of Agreement / Disagreement

Participants express differing views on the derivation of the characteristic function and its comparison to moment-generating functions. There is no consensus on the necessity of its complex-valued nature or its derivation process.

Contextual Notes

Some participants highlight limitations in the applicability of moment-generating functions, noting that not all distributions have them, which contrasts with the universal existence of characteristic functions.

Who May Find This Useful

This discussion may be useful for students and practitioners in probability theory and statistics, particularly those interested in the properties and applications of characteristic functions and moment-generating functions.

cappadonza
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so a charateristic function of a RV is complex valued funtion. from my lecture, the distribution funtion of a Random variable is not always "well behaved", may not have a density etc. A charateristic function on the other had is "well behave".
What i don't understand is, is that the only reason we use it ?
how is it actually derived, why does it have to be complex valued .
this is the definition I'm given [tex]\phi(t) = \mathbb{E}(e^{itX})[/tex]
how is this actually derived, is somewhere where i can find the proof ?
 
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You need to clarify your question. Definitions aren't derived.

As far as usage, the simplest example is deriving the distribution function a sum of independent random variables. The characteristic function of the sum is the product of the characteristic functions of the individual variables.
 
if you are studying characteristic functions you should have already seen moment-generating functions.

moment generating functions can be used to uniquely identify the form of a distribution IF (big if) the moments of the distribution satisfy a very strict requirement. that doesn't happen all the time.

even worse, not every probability distribution has a moment-generating function: think of a t-distribution with 5 degrees of freedom: no moments of order 4 or greater, so no moment generating function.

however, EVERY distribution has a characteristic function, and every distribution is uniquely determined by the form of that function. that is one (not the only) reason for their importance.
 
I found this presentation to be helpful in understanding the derivation and applications of the characteristic function.

http://www.sjsu.edu/faculty/watkins/charact.htm
 

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