We describe the general form of surjective maps on the cone of all positive operators which preserve order and spectrum. The result is optimal as shown by counterexamples. As an easy consequence we characterize surjective order and spectrum preserving maps on the set of all self-adjoint operators.
COBISS.SI-ID: 18189145
We briefly survey some recent results describing the general form of various symmetries of effect algebras. In particular, we outline some of the proof techniques and list a few most interesting open problems. A finite-dimensional version of classical Ludwig's characterization of ortho-order automorphisms of effect algebras is proved in the absence of the bijectivity assumption.
COBISS.SI-ID: 18607961
We prove that every continuous map acting on the four-dimensional Minkowski space and preserving light cones in one direction only is either a Poincaré similarity, that is, a product of a Lorentz transformation and a dilation, or it is of a very special degenerate form.
COBISS.SI-ID: 18656857
The known descriptions of the groups of order automorphisms of operator intervals are very simple with only one exception: there are two known results describing the general form of order automorphisms of the effect algebra and they both look quite complicated. It is the aim of this paper to show that the group of order automorphisms of the effect algebra is isomorphic to the group of order automorphisms of any other proper operator interval. After proving this statement we will present a new description of order automorphisms of the effect algebra explaining better that the case of the effect algebra is not more complicated than the other operator intervals.
COBISS.SI-ID: 18385241
Botelho, Jamison, and Molnár , and Gehér and Šemrl have recently described the general form of surjective isometries of Grassmann spaces of all projections of a fixed finite rank on a Hilbert space $H$. As a straightforward consequence one can characterize surjective isometries of Grassmann spaces of projections of a fixed finite corank. In this paper we solve the remaining structural problem for surjective isometries on the set $P_\infty (H)$ of all projections of infinite rank and infinite corank when $H$ is separable. The proof technique is entirely different from the previous ones and is based on the study of geodesics in the Grassmannian $P_\infty (H)$. However, the same method gives an alternative proof in the case of finite rank projections.
COBISS.SI-ID: 18384473