Multiplication Property of Characteristic Functions
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Theorem
Let $X$ and $Y$ be independent random variables on a probability space $\struct {\Omega, \Sigma, \Pr}$.
Let $\phi_X$ and $\phi_Y$ denote the characteristic functions of $X$ and $Y$ respectively.
Then:
- $\phi_{X + Y} = \phi_X \phi_Y$
Proof
Let $i = \sqrt{-1}$.
Let $\expect X$ denote the expectation of $X$.
\(\ds \map {\phi_{X + Y} } t\) | \(=\) | \(\ds \expect {e^{i t \paren {X + Y} } }\) | Definition of Characteristic Function of Random Variable | |||||||||||
\(\ds \) | \(=\) | \(\ds \expect {e^{i t X} e^{i t Y} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \expect {e^{i t X} } \expect {e^{ i t Y} }\) | Functions of Independent Random Variables are Independent, Expected Value of Product is Product of Expected Value | |||||||||||
\(\ds \) | \(=\) | \(\ds \map {\phi_X} t \map {\phi_Y} t\) |
Hence:
- $\phi_{X + Y} = \phi_X \phi_Y$
$\blacksquare$
Sources
- 2005: Neil A. Weiss: A Course in Probability: $\S 11.1$