This is solely a reference request. I have heard a few versions of the following theorem:
If the joint moment generating function $\mathbb{E}[e^{uX+vY}] = \mathbb{E}[e^{uX}]\mathbb{E}[e^{vY}]$ whenever the expectations are finite, then $X,Y$ are independent.
And there is a similar version for characteristic functions.
Could anyone provide me a serious reference which proves one or both of these theorems?
Best Answer
Proof:
Remark: It is not important that $X$ and $Y$ are vectors of the same dimension. The same reasoning works if, say, $X$ is an $\mathbb{R}^k$-valued random variable and $Y$ and $\mathbb{R}^d$-valued random variable.
Reference (not for the given proof, but the result):David Applebaum, B.V. Rajarama Bhat, Johan Kustermans, J. Martin Lindsay, Michael Schuermann, Uwe Franz: Quantum Independent Increment Processes I: From Classical Probability to Quantum Stochastic Calculus (Theorem 2.1).