Step unclear in Peebbles' Probability Book

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SUMMARY

The discussion centers on understanding a specific step in Peebles' Probability Book related to autocorrelation functions. Participants clarify the transition from a product in the expectation value to a sum, specifically in the context of calculating the autocorrelation function \( R_{X,X}(t,t+\tau) \). The formula \( \cos(a) \cos(b) = \frac{1}{2}(\cos(a+b) + \cos(a-b)) \) is identified as a critical identity that facilitates this transformation. This insight resolves the confusion regarding the mathematical manipulation involved.

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  • Familiarity with expectation values in statistical analysis
  • Knowledge of trigonometric identities, specifically the cosine product-to-sum formula
  • Basic concepts of signal processing and time series analysis
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Students and researchers in probability theory, statisticians, and anyone studying signal processing who seeks to deepen their understanding of autocorrelation and expectation values.

snatchingthepi
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TL;DR
There's a step from one line to another that isn't clear to me.
If anyone could help me understand how Peebles gets from line one of the autocorrelation to the second line, I'd be most grateful. I don't understand what identity or property is being used to go from a product in the expectation value to a sum in the expectation value.
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I'm not sure I understand the step you are struggling with. Is it where they calculate ##R_{X,X}(t,t+\tau)##?

In that case, the formula

$$\cos(a) \cos(b)= 1/2(\cos(a+b)+\cos(a-b))$$ can be useful.
 
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Sorry for the lateness of my thanks, but #gradschool. Yeah I feel slightly stupid now for not realizing that. It was exactly what I needed. Thank you.
 
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