What is the average value of a bounded periodic function over a period?

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Discussion Overview

The discussion revolves around the concept of the average value of a bounded periodic function, particularly focusing on the implications of specifying the range over which the average is calculated. Participants explore the conditions under which average values are defined and the relevance of periodicity in this context.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the validity of discussing the average value of a function without specifying the range, using the example of the complex exponential function ##e^{ix}##, which averages to zero only over a specific interval of ##2\pi##.
  • Another participant agrees that while the domain is often implicit in discussions, it is important to specify it, especially in cases like sine waves.
  • A third participant asserts that for periodic functions, it is reasonable to refer to the average over one period as the average of the function.
  • A further perspective suggests that the average of a bounded periodic function can also be approached by taking the limit of its average over an increasingly large domain, proposing a method to define the average without knowing the specific period.

Areas of Agreement / Disagreement

Participants exhibit some agreement on the importance of specifying the range for average calculations, particularly in the context of periodic functions. However, there are differing views on how to approach the definition of average values, indicating that multiple competing perspectives remain in the discussion.

Contextual Notes

There are unresolved assumptions regarding the definitions of boundedness and periodicity, as well as the implications of taking limits over larger domains. The discussion does not clarify how these factors might influence the average value calculations.

kent davidge
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Does it make sense to just talk about the average value of a function without specifying the range over which the average is taken? It seems a common occurrence in discussions of waves to just mention that the average value of the complex exponential ##e^{ix}## is zero. But it will be zero only if we look at it over a ##2\pi## interval, like from ##-\pi## to ##\pi##, correct?
 
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Your asertion is correct. However in many instances, the domain is obvious, so it is not stated explicitly, such as one period for sine waves, etc.
 
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If a function is periodic then it makes sense to refer to the average over one period as the average of the function.
 
Another way to view the average of a bounded periodic function is to take the limit of its average over an increasingly large domain. So that if, say, f : RR is a bounded continuous function with some period p > 0, so that f(x + p) = f(x) for all x, then the limit as T → ∞ of 1/(2T) times the integral of f(x) over the interval [-T, T] will approach the same value as its average over one full period.

So if we know f is a bounded continuous periodic function, we can define its average over a period without knowing what that period is.
 
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