Communicated by E. Hewitt, July 29, 1966 | BY P. PORCELLI AND E. A. PEDERSEN
This paper, authored by P. Porcelli and E. A. Pedersen, explores the properties of rings of operators in a complex Hilbert space \(\mathcal{H}\). The authors focus on an abelian symmetric subring \(E\) of the ring of bounded operators \(B(\mathcal{H})\), which is closed in the weak operator topology and contains the identity. They show that every linear functional \(T\) on \(E\) is continuous in the weak operator topology, leading to the conclusion that \(T\) is continuous in all four topologies: weak, ultraweak, strong, and ultrastrong. This result is used to derive an integral representation for such functionals and to prove a theorem on centrally reducible positive functionals on \(E_1\), the commutant of \(E\).
The maximal ideal space \(M\) of \(E\) is defined, and the Gel'fand transforms of elements in \(E\) are introduced. The paper establishes that every bounded linear functional on \(E\) can be represented as an integral with respect to a unique complex Borel measure \(\nu\) on \(M\). Special attention is given to functionals of the form \(A \mapsto (A\xi, \xi)\), where \(\xi\) is a vector in \(\mathcal{H}\). The measures \(\nu_\xi\) corresponding to these functionals are shown to be nonnegative and absolutely continuous with respect to \(\mu\), the measure corresponding to the normalized vector \(\xi_0\).
The paper includes a theorem stating that a linear functional \(T\) on \(E\) is ultrastrongly continuous if and only if it is weakly continuous, and if and only if there exists a \(\phi \in L_1(M, \mu)\) such that \(T(A) = \int_M a(m) \phi(m) d\mu(m)\) for every \(A \in E\). The proof of this theorem involves showing that ultrastrong continuity implies weak continuity, and vice versa, using the properties of the measures and the Gel'fand transforms.
Additionally, the paper discusses the concept of centrally reducible positive functionals on \(E_1\) and proves that if \(T_1\) is a weakly continuous positive functional on \(E_1\), then it is centrally reducible. The proof involves showing that the restriction of \(T_1\) to \(E\) is weakly continuous and can be represented by an integral, and then using this representation to show that any positive functional \(T'\) on \(E\) that is dominated by \(T\) can be expressed as \(T'(A) = T(AB)\) for some \(B \in E\).This paper, authored by P. Porcelli and E. A. Pedersen, explores the properties of rings of operators in a complex Hilbert space \(\mathcal{H}\). The authors focus on an abelian symmetric subring \(E\) of the ring of bounded operators \(B(\mathcal{H})\), which is closed in the weak operator topology and contains the identity. They show that every linear functional \(T\) on \(E\) is continuous in the weak operator topology, leading to the conclusion that \(T\) is continuous in all four topologies: weak, ultraweak, strong, and ultrastrong. This result is used to derive an integral representation for such functionals and to prove a theorem on centrally reducible positive functionals on \(E_1\), the commutant of \(E\).
The maximal ideal space \(M\) of \(E\) is defined, and the Gel'fand transforms of elements in \(E\) are introduced. The paper establishes that every bounded linear functional on \(E\) can be represented as an integral with respect to a unique complex Borel measure \(\nu\) on \(M\). Special attention is given to functionals of the form \(A \mapsto (A\xi, \xi)\), where \(\xi\) is a vector in \(\mathcal{H}\). The measures \(\nu_\xi\) corresponding to these functionals are shown to be nonnegative and absolutely continuous with respect to \(\mu\), the measure corresponding to the normalized vector \(\xi_0\).
The paper includes a theorem stating that a linear functional \(T\) on \(E\) is ultrastrongly continuous if and only if it is weakly continuous, and if and only if there exists a \(\phi \in L_1(M, \mu)\) such that \(T(A) = \int_M a(m) \phi(m) d\mu(m)\) for every \(A \in E\). The proof of this theorem involves showing that ultrastrong continuity implies weak continuity, and vice versa, using the properties of the measures and the Gel'fand transforms.
Additionally, the paper discusses the concept of centrally reducible positive functionals on \(E_1\) and proves that if \(T_1\) is a weakly continuous positive functional on \(E_1\), then it is centrally reducible. The proof involves showing that the restriction of \(T_1\) to \(E\) is weakly continuous and can be represented by an integral, and then using this representation to show that any positive functional \(T'\) on \(E\) that is dominated by \(T\) can be expressed as \(T'(A) = T(AB)\) for some \(B \in E\).