Approximation of transfer functions of infinite dimensional dynamical systems by rational interpolants with prescribed poles

28 pages, 3 figures.-- MSC2000 codes: 41A20, 30E10, 41A05.Rational interpolants, some of the poles of which are left free, are used to approximate the transfer functions of a large class of possibly unstable infinite-dimensional control systems. The free poles are used to detect the singularities of the transfer function which are responsible for the instability of the system.Publicad