The addition and multiplication theorems

Abstract: We discuss the classes $\fC$, $\fM$, and $\fS$ of analytic functions that can be realized as the Liv\v{s}ic characteristic functions of a symmetric densely defined operator $\dot A$ with deficiency indices $(1,1)$, the Weyl-Titchmarsh functions associated with the pair $(\dot A, A)$ where $A$ is a self-adjoint extension of $\dot A$, and the characteristic function of a maximal dissipative extension $\widehat A$ of $\dot A$, respectively. We show that the class $\fM$ is a convex set, both of the classes $\fS$ and $\fC$ are closed under multiplication and, moreover, $\fC\subset \fS$ is a double sided ideal in the sense that $\fS\cdot \fC=\fC\cdot \fS\subset \fS$. The goal of this paper is to obtain these analytic results by providing explicit constructions for the corresponding operator realizations. In particular, we introduce the concept of an operator coupling of two unbounded maximal dissipative operators and establish an analog of the Liv\v{s}ic-Potapov multiplication theorem [14] for the operators associated with the function classes $\fC$ and $\fS$. We also establish that the modulus of the von Neumann parameter characterizing the domain of $\widehat A$ is a multiplicative functional with respect to the operator coupling.