The open-loop transfer function of a unity feedback control system is given by G(s) = k / (s+2)(s+4)(s2+6s+25).
By applying Routh-Hurwitz criterion, determine (i) the range of k for which the closed-loop system will be stable and (ii) the values of k which will cause sustained oscillations in the closed-loop system. What are the corresponding oscillation frequencies?