Reflexive operator algebra explained

In functional analysis, a reflexive operator algebra A is an operator algebra that has enough invariant subspaces to characterize it. Formally, A is reflexive if it is equal to the algebra of bounded operators which leave invariant each subspace left invariant by every operator in A.

This should not be confused with a reflexive space.

Examples

Nest algebras are examples of reflexive operator algebras. In finite dimensions, these are simply algebras of all matrices of a given size whose nonzero entries lie in an upper-triangular pattern.

In fact if we fix any pattern of entries in an n by n matrix containing the diagonal, then the set of all n by n matrices whose nonzero entries lie in this pattern forms a reflexive algebra.

An example of an algebra which is not reflexive is the set of 2 × 2 matrices

\left\{ \begin{pmatrix} a&b\ 0 & a \end{pmatrix} : a,b\in C \right\}.

This algebra is smaller than the Nest algebra

\left\{ \begin{pmatrix} a&b\ 0 & c \end{pmatrix} : a,b,c\in C \right\}

but has the same invariant subspaces, so it is not reflexive.

If T is a fixed n by n matrix then the set of all polynomials in T and the identity operator forms a unital operator algebra. A theorem of Deddens and Fillmore states that this algebra is reflexive if and only if the largest two blocks in the Jordan normal form of T differ in size by at most one. For example, the algebra

\left\{ \begin{pmatrix} a & b & 0 \ 0 & a & 0 \ 0 & 0 & a \end{pmatrix} : a,b\in C \right\}

which is equal to the set of all polynomials in

T = \begin{pmatrix} 0 & 1 & 0 \ 0 & 0 & 0 \ 0 & 0 & 0 \end{pmatrix}

and the identity is reflexive.

Hyper-reflexivity

Let

l{A} be a weak*-closed operator algebra contained in B(H), the set of all bounded operators on a Hilbert space H and for T any operator in B(H), let\beta(T, l{A})=\sup \left\{ \left\| P ^{\perp} TP \right\| : P isaprojectionand P ^{\perp} l{A} P = ( 0 ) \right\} .

Observe that P is a projection involved in this supremum precisely if the range of P is an invariant subspace of

l{A} .

The algebra

l{A} is reflexive if and only if for every T in B(H):\beta(T, l{A})=0 impliesthat T isin l{A} .

We note that for any T in B(H) the following inequality is satisfied:

\beta(T, l{A})\le dist (T, l{A}) .

Here

dist (T, l{A}) is the distance of T from the algebra, namely the smallest norm of an operator T-A where A runs over the algebra. We call l{A} hyperreflexive if there is a constant K such that for every operator T in B(H),dist (T, l{A})\le K \beta(T, l{A}) .

The smallest such K is called the distance constant for

l{A} . A hyper-reflexive operator algebra is automatically reflexive.

In the case of a reflexive algebra of matrices with nonzero entries specified by a given pattern, the problem of finding the distance constant can be rephrased as a matrix-filling problem: if we fill the entries in the complement of the pattern with arbitrary entries, what choice of entries in the pattern gives the smallest operator norm?

Examples

• Every finite-dimensional reflexive algebra is hyper-reflexive. However, there are examples of infinite-dimensional reflexive operator algebras which are not hyper-reflexive.
• The distance constant for a one-dimensional algebra is 1.
• Nest algebras are hyper-reflexive with distance constant 1.
• Many von Neumann algebras are hyper-reflexive, but it is not known if they all are.
• A type I von Neumann algebra is hyper-reflexive with distance constant at most 2.

See also

• Invariant subspace
• subspace lattice
• reflexive subspace lattice
• nest algebra

References

• William Arveson, Ten lectures on operator algebras,
• H. Radjavi and P. Rosenthal, Invariant Subspaces,