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Let H be a Hilbert $C^*$-module over a matrix algebra A. It is proved that any function $T:H\to H$ which preserves the absolute value of the (generalized) inner product is of the form $Tf=\phi(f)Uf$ $(f\in H)$, where $\phi$ is a phase-function and U is an A-linear isometry. The result gives a natural extension of Wigner's classical unitary-antiunitary theorem for Hilbert modules., Comment: 27 pages. To appear in J. Math. Phys

We present the following reflexivity-like result concerning the automorphism group of the $C^*$-algebra B(H), H being a separable Hilbert space. Let $\phi:B(H)\to B(H)$ be a multiplicative map (no linearity or continuity is assumed) which can be approximated at every point by automorphisms of B(H) (these automorphisms may, of course, depend on the point) in the operator norm. Then $\phi$ is an automorphism of the algebra B(H)., Comment: 8 pages. To appear in Acta Sci. Math. (Szeged)

In this paper we describe the form of those continuous multiplicative maps on B(H) (H being a separable complex Hilbert space of dimension not less than 3) which preserve the rank, or the corank. Furthermore, we characterize those continuous *-semigroup endomorphisms of B(H) which are spectrum non-increasing., Comment: 13 pages. To appear in Linear Algebra Appl

As a continuation of the work on linear maps between operator algebras which preserve certain subsets of operators with finite rank, or corank, here we consider the problem inbetween, that is, we treat the question of preserving operators with infinite rank and infinite corank. Since, as it turns out, in this generality our preservers cannot be written in a nice form what we have got used to when dealing with linear preserver problems, hence we restrict our attention to certain important classes of operators like idempotents, or projections, or partial isometries. We conclude the paper with a result on the form of linear maps which preserve the left ideals in B(H)., Comment: 11 pages. To appear in Linear Algebra Appl

We present an operator algebraic approach to Wigner's unitary-antiunitary theorem using some classical results from ring theory. To show how effective this approach is, we prove a generalization of this celebrated theorem for Hilbert modules over matrix algebras. We also present a Wigner-type result for maps on prime C*-algebras., Comment: AMSLaTeX, 17 pages. To appear in J. Austral. Math. Soc

We show that with few exceptions every local isometric automorphism of the group algebra $L^p(G)$ of a compact group $G$ is an isometric automorphism., Comment: To appear in Proc. AMS

The aim of this paper is to show that the automorphism and isometry groups of the suspension of $B(H)$, $H$ being a separable infinite dimensional Hilbert space, are algebraically reflexive. This means that every local automorphism, respectively local surjective isometry of $C_0(\mathbb R)\otimes B(H)$ is an automorphism, respectively a surjective isometry.

The aim of this paper is to solve a linear preserver problem on the function algebra $C(X)$. We show that in case $X$ is a first countable compact Hausdorff space, every linear bijection $\phi:C(X)\to C(X)$ having the property that $diam(\phi(f)(X))=diam(f(X))$ $(f\in C(X))$ is of the form \[ \phi(f)=\tau \cdot f\circ \varphi +t(f)1 \qquad (f\in C(X)) \] where $\tau$ is a complex number of modulus 1, $\varphi:X\to X$ is a homeomorphism and $t$ is a linear functional on $C(X)$.

The aim of this note is to show that the automorphism and isometry groups of the C*-algebra $\l_\infty(N,B(H))$ of all bounded sequences in $B(H)$ are topologically reflexive which, as one of our former results shows, is not the case with the "scalar algebra" $l_\infty$.

In this paper we give an example of a proper standard C*-algebra (a proper C*-subalgebra of B(H) containing C(H)) whose automorphism and isometry groups are topologically reflexive. Furthermore, we prove that in the case of extensions of C(H) by separable commutative C*-algebras, these groups are algebraically reflexive. Concerning the most well-known extensions of C(H) by the algebra of all continuous complex valued functions on the perimeter of the unit disc, we show that the automorphism and isometry groups are topologically nonreflexive.

The aim of this paper is to characterize those linear maps from a von Neumann factor $\A$ into itself which preserve the extreme points of the unit ball of $\A$. For example, we show that if $\A$ is infinite, then every such linear preserver can be written as a fixed unitary operator times either a unital *-homomorphism or a unital *-antihomomorphism.