On bi-free multiplicative convolution
Volume 248 / 2019
Abstract
We study the partial bi-free $S$-transform of a pair $(a,b)$ of random variables, and the $S$-transform of the $2\times 2$ matrix-valued random variable $(\begin{smallmatrix}a&0\\0&b\end{smallmatrix})$ associated with $(a,b)$ when restricted to upper triangular $2\times 2$ matrices. We first derive an explicit expression for bi-free multiplicative convolution (of probability measures on the 2-dimensional torus $\mathbb{T}^2=\{(s,t)\in\mathbb{C}^2:|s|=1=|t|\}$, or on $\mathbb{R}^2_+$ in $\mathbb{C}^2$) from a subordination equation for bi-free multiplicative convolution. We then show that, when $(a_1, b_1)$ and $(a_2,b_2)$ are bi-free, the $S$-transforms of $X_1=(\begin{smallmatrix}a_1&0\\0&b_1\end{smallmatrix})$, $X_2=(\begin{smallmatrix}a_2&0\\0&b_2\end{smallmatrix})$ satisfy Dykema’s twisted multiplicative equation for free operator-valued random variables if and only if at least one of the two partial bi-free $S$-transforms of the pairs of random variables is the constant function 1 in a neighborhood of $(0,0)$. This is the case if and only if one of the two pairs, say $(a_1,b_1)$, has factoring two-band moments (that is, $\varphi(a_1^mb_1^n)=\varphi(a_1^m)\varphi(b_1^n)$ for all $m,n=1, 2, \ldots$). We thus find a lot of bi-free pairs of random variables for which the $S$-transforms of the corresponding matrix-value random variables do not satisfy Dykema’s twisted multiplicative formula. Finally, if both $(a_1,b_1)$ and $(a_2,b_2)$ have factoring two-band moments, we prove that the $\Psi$-transforms of $X_1$, $X_2$, and $X_1X_2$ satisfy a subordination equation.
