A class of marked invariant subspaces with an application to algebraic   Riccati equations
  Invariant subspaces of a matrix $A$ are considered which are obtained by truncation of a Jordan basis of a generalized eigenspace of $A$. We characterize those subspaces which are independent of the choice of the Jordan basis. An application to Hamilton matrices and algebraic Riccati equations is given. 
Pudji Astuti, Harald K. Wimmer
math.RA