Paper detail

Hermitian geometry on resolvent set(I)

For a tuple $A=(A_1,\ A_2,\ ...,\ A_n)$ of elements in a unital Banach algebra ${\mathcal B}$, its projective joint spectrum $P(A)$ is the collection of $z\in {\mathbb C}^n$ such that $A(z)=z_1A_1+z_2A_2+\cdots +z_nA_n$ is not invertible. It is known that the ${\mathcal B}$-valued $1$-form $ω_A(z)=A^{-1}(z)dA(z)$ contains much topological information about the joint resolvent set $P^c(A)$. This paper studies geometric properties of $P^c(A)$ with respect to Hermitian metrics defined through the ${\mathcal B}$-valued {\em fundamental form} $Ω_A=-ω^*_A\wedge ω_A$ and its coupling with faithful states $ϕ$ on ${\mathcal B}$, i.e. $ϕ(Ω_A)$. The connection between the tuple $A$ and the metric is the main subject of this paper. In particular, it shows that the Kählerness of the metric is tied with the commutativity of the tuple, and its completeness is related to the Fuglede-Kadison determinant.