According to Mayne’s paper "Robust Model Predictive Control of Constrained Linear Systems with Bounded Disturbances", when considering a nonzero initial error, i.e., x∈x0⊕Z, the computation of ZZ is referenced. The paper suggests that ZZ can be determined based on the approach in "Invariant Approximations of the Minimal Robust Positively Invariant Set". However, the latter assumes a zero initial condition.