Department of Mathematics, University of Zagreb, Bijenicka c. 30, 10000 Zagreb, Croatia ; Department of Mathematics, University of Zagreb, Bijenicka c. 30, 10000 Zagreb, Croatia
Abstract:
Let be a Hilbert -module over the -algebra of all compact operators on a Hilbert space. It is proved that any function which preserves the absolute value of the -valued inner product is of the form , where is a phase function and is an -linear isometry. The result generalizes Molnár's extension of Wigner's classical unitary-antiunitary theorem.