Singular fibres of the Gelfand–Cetlin system on MANI( <i>n</i> )*
Singular fibres of the Gelfand–Cetlin system on MANI( <i>n</i> )*
In this paper, we show that every singular fibre of the Gelfand-Cetlin system on co-adjoint orbits of unitary groups is a smooth isotropic submanifold which is diffeomorphic to a two-stage quotient of a compact Lie group by free actions of two other compact Lie groups. In many cases, these singular …