The phrase "are not unstable" is correct and usable in written English.
You can use it when describing something that is stable or reliable, indicating that it does not exhibit instability.
Example: "The results of the experiment are not unstable, which suggests that the methodology was sound."
Alternatives: "are stable" or "are reliable".
7. (S) If Israel's neighborhood were not unstable enough, Dagan observed, it did not help that Russia is playing a "very negative role" in the region.
Under Saddam Hussein, Iraq may have had a brutish existence, but it was not "unstable," or indeed going anywhere before our invasion (that is, it was successfully contained under Clinton-era policies), despite a decade of sanctions and no-flight zones over two-thirds of the country.
One anomaly is that there can be two realizations, both approximately reachable and observable, but one is exponentially stable and the other is not (unstable).
Rarely described 3-part models were synthetic, or were not unstable in the fracture line between the greater tuberosity and the humeral head (Ruch et al. 2000, Sanders et al. 2007).
When the economy falls, gold tends to rise because it isn't unstable like the money it backs.
God's existence and character, on the other hand, are not insecure, unstable, or transient; in a word, they are not contingent.
Systems with simultaneous input and state delay have remained a challenge, although exponential stabilization has been solved for systems that are not exponentially unstable, such as chains of delayed integrators and systems in the 'feedforward' form.
Tumour blood vessels show many differences from normal vessels and are not genetically unstable, so they form a potentially key area for therapy of all types of cancer including leukaemias.
However, problems in oil drilling, combustion dynamics, and other applications give rise to wave PDE models, which are not only unstable, but have all of their infinitely many eigenvalues at uncertain locations in the right half plane (anti-stable).
