  Let group generators having finite-dimensional representation be realized as
Hermitian linear differential operators without nhomogeneous terms as takes
place, for example, for the SO(n) group. Then orresponding group Hamiltonians
containing terms linear in generators (along with quadratic ones) give rise to
quasi-exactly solvable models with a magnetic field in a curved space. In
particular, in the two-dimensional case such models are generated by quantum
tops. In the three-dimensional one for the SO(4) Hamiltonian with an isotropic
quadratic part the manifold within which a quantum particle moves has the
geometry of the Einstein universe.
