The controllers presented in log post [11731], while leading to a stable closed loop system, are not in themselves stable. The proper term for this is they are not "strongly stable." This does not affect their performance and they are the optimal controller however it may be harder to implement unstable controllers. In this case it may be helpful to find a way to ensure that the final system is strongly stable. There are a few papers on this by various authors on the topic with two being the most useful because they are by authors that we used to write buzz's \(H_2\) /\(H_\infty\) code. The first is \(H_2\) and Mixed  \(H_2\)/ \(H_\infty\) Stable Stabilization (Google Scholar) which lays out the fixed order solution for \(L_2\) disturbances with a fixed \(H_\infty\) performance bound in section 5. The second is Stable Stabilization with \(H_2\) and \(H_\infty\) Performance Constraints (Google Scholar) which gives a more generalized solution for strongly stable controllers. While the second paper says that it is giving a mixed norm result, I'm not sure it is. I would call mixed norm a minimization of \(||H_2|| + ||H_\infty||\) which is diffrent than our goal of minimizing \(||H_2||\) given a \(H_\infty\) bound. Given the papers language and references to its first reference which is about LQG with an \(H_\infty\) bound, I think in reality it is not a mixed norm bit in fact \(||H_2||\) given a \(H_\infty\) bound.