A counterexample for positive realization problem

Gives a counterexample for positive realization of impulse response. The positive realization problem is to find a state space model x(k+1)=Ax(k)+bu(k), y(k)=c/sup T/ x(k) from a given impulse response H/sub k/, k=1, 2, ... of transfer function G(z), in that all the elements of A, b, and c are restricted to be nonnegative real numbers. Such problem occurs in modeling compartmental systems and in the hidden Markov process.