Operator Geometry of Hilbert Ball Automorphisms

Abstract

We consider the operator--theoretic model for the group of biholomorphic automorphisms $Aut(B)$ of the unit ball $B$ of a complex Hilbert space $\H$ by representing each automorphism as a bounded linear operator on the augmented Hilbert space $\H\oplus \mathbb{C}$. Any member of $Aut(B)$ admits a natural block operator matrix representation acting on $\mathcal{H}\oplus\mathbb{C}$. We study the geometry of the subset $M(\mathcal{H})$ of $\mathcal{B}(\mathcal{H}\oplus\mathbb{C})$ consisting of these block operator matrices. It is shown that every element corresponding to a non-rotation automorphism is a smooth point of $\mathcal{B}(\mathcal{H}\oplus\mathbb{C})$. Orthogonality between two such matrices is characterized geometrically by the antipodality of the corresponding Möbius images of a boundary point of the ball. This orthogonality characterization is applied to show that an inner automorphism of $\Aut(B)$ that preserves Birkhoff--James orthogonality in both directions if and only if it is conjugation by a pure rotation, yielding a rigidity result. The normalized block matrices are $J$-unitary under a suitable normalization, where $J = \operatorname{diag}(I_{\mathcal{H}}, -1)$. We show that norm of such block matrices satisfy a submultiplicativity under a certain composition rule other than usual operator multiplication, and induce a metric on certain subsets of $Aut(B)$ which recover the hyperbolic metric on the Hilbert ball. The symmetric structure of Birkhoff--James orthogonality within $M(\mathcal{H})$ is also studied: there are no left-symmetric points, while the only right-symmetric points are pure rotations.