A Direct Proof of a Theorem Concerning Singular Hamiltonian Systems
  This technical report presents a direct proof of Theorem~1 in [1] and some consequences that also account for (20) in [1]. This direct proof exploits a state space change of basis which replaces the coupled difference equations (10) in [1] with two equivalent difference equations which, instead, are decoupled. 
Giovanni Marro
cs.SY