y condition is that
  rank([ B'  L'  R']') = m.

  Under these assumptions the algebraic Riccati equation is known to
  have a unique non-negative definite solution.
  The first step in the method of deflating subspaces is to form the
  extended matrices in (3), of order 2N + M. Next, these pencils are
  compressed to a form of order 2N (see [1])

     lambda x A  - B .
               f    f

  This generalized eigenvalue problem is then solved using the QZ
  algorithm and the stable deflating subspace Ys i