[1] C. A. Desoer C. T. Chen: Cotrollability and observability of feedback systems. IEEE Trans. Aut. Control AC-12 (1967), 4, 474-475.
[2] Ф. P. Гантмахер: 
Теория матрия. Наука, Москва 1966. 
Zbl 1155.78304[3] C. T. Chen: Stability of linear multivariable feedback systems. Proceedings IEEE 56 (1968), 5, 821-828.
[4] R. E. Kalman: 
Irreducible realizations and the degree of a rational matrix. Journal SIAM 13 (1965), 2, 520-544. 
MR 0182479 | 
Zbl 0161.06704[5] I. W. Sandberg: 
On the theory of linear multi-loop feedback systems. The Bell System Technical Journal 42 (1963), 355-382. 
MR 0149988