A system with the characteristic equation s4 + 2s3 + 11s2

A system with the characteristic equation s4 + 2s3 + 11s2 + 18s + 18 = 0 will have closed loop poles such that—

1. all poles lie in the left half of the s-plane
2. all poles lie in the right half of the s-plane
3. two poles lie symmetrically on the imaginary axis of the s-plane
4. no pole lie on the imaginary axis of the s-plane

Routh Hurwitz criterion of the given equation
Zero is first column at s1. Hence two poles lie symmetrically on the imaginary axis of the s-plane.