Exactness of the upper robustness bound for some norm-bounded disturbances

Let $\overline{\nu}$ denote the paper's convex upper bound for the robustness measure $\nu$. A disturbance is norm-bounded if it belongs to a class constrained by a specified norm.

Exactness conjecture. $\overline{\nu}$ is exact for some class of norm-bounded disturbances.

The preceding analysis shows that the upper bound can have a substantial gap from $\nu$, while the pure ring example attains the factor-$n$ bound. The source asserts existence of a class where the bound is exact but does not identify that class in the conjecture itself.