QR-hypersurfaces of quaternionic Kähler manifolds.
Quadric hypersurfaces of finite type
Quasi-minimal rotational surfaces in pseudo-Euclidean four-dimensional space
In the four-dimensional pseudo-Euclidean space with neutral metric there are three types of rotational surfaces with two-dimensional axis - rotational surfaces of elliptic, hyperbolic or parabolic type. A surface whose mean curvature vector field is lightlike is said to be quasi-minimal. In this paper we classify all rotational quasi-minimal surfaces of elliptic, hyperbolic and parabolic type, respectively.