From WIKIPEDIA
"In the theory of dynamical systems, and control theory, a continuous linear time-invariant system is marginally stable if and only if the real part of every eigenvalue (or pole) in the system's transfer-function is non-positive, and all eigenvalues with zero real value are simple roots (i.e. the eigenvalues on the imaginary axis are all distinct from one another). If all the poles have strictly negative real parts, the system is instead asymptotically stable.
A discrete linear time-invariant system is marginally stable if and only if the transfer function's spectral radius is 1. That is, the greatest magnitude of any of the eigenvalues (or poles) of the transfer function is 1. The values of the poles must also be distinct. If the spectral radius is less than 1, the system is instead asymptotically stable."
Stability is "stable" when there is imperfection and this contradicts what Six Sigma tries to do from a system control perspective. It is a certain derangement that creates stability.
