*Wissenschaftliches Programm*   *Liste der Vortragenden*

Sektion 4
Dienstag, 19.09.2000, 17.00–17.20 Uhr, POT 251

## Completely contractive representations of certain commutative algebras of matrices

#### Ralf Meyer, Universität Münster

Given an operator algebra A and a representation  of A on a Hilbert space, it is usually quite difficult to decide (numerically) whether  is completely contractive, even if A is a commutative algeba of matrices and  is finite dimensional. We consider a situation where there is an easy-to-check criterion for a representation to be completely contractive.

A d-tuple of commuting elements (T j) of A is called a d-contraction iff the row matrix (T 1 . . . T d) is a contraction. We say that a commutative operator algebra A is determined by (T j) iff the following holds: A representation  of A is completely contractive iff (T j) is a d-contraction.

We will consider the case where A is a commutative, d-dimensional subalgebra of the algebra Md of d × d-matrices. Using Arveson’s dilation theory for d-contractions, we obtain an algorithm to decide whether a given subalgebra A is determined by a d - 1-contraction (T j) and to compute (T j).

Endow the set of all commutative d-dimensional subalgebras of Md with the subspace topology from the Grassmannian manifold. It turns out that the set of A that are determined by some d - 1-contraction has non-empty interior. Hence the set of “well-behaved” subalgebras is not too small.

It turns out that a commutative, d-dimensional subalgebra of the algebra of d × d-matrices has a good chance to be determined by some d - 1-contraction.

If we topologize the set of d-dimensional, commutative subalgebras with the topology of the Grassmannian manifold, the set of algebras that are determined by a d - 1-contraction has non-empty interior.