Beyond Observability: Sharp Sample Bounds for Minimax State Reconstruction

Overview & Synopsis

We study how many output samples match the worst-case state reconstruction error obtainable from a continuous history under bounded measurement noise. For observable scalar-output linear time-varying systems with $n$ states, $n$ samples suffice for exact reconstruction, whereas at most $n^2$ attain the continuous-history minimax error in the $\ell_\infty$ norm. An explicit construction proves sharpness even for real-analytic systems. For time-invariant systems, strict total positivity yields one $n$-sample schedule optimal for every state coordinate; endpoint-observed tridiagonal cooperative chains satisfy this conclusion in physical coordinates. Interior sensing behaves differently: an explicit three-state family requires four samples despite positivity and distinct real eigenvalues. The strict three-state gap persists under small stabilizing damping. A nilpotent four-state example and an interval proof for a six-state mechanical system give gaps. The results identify how temporal sampling and the prescribed state-error geometry determine the measurements required for minimax reconstruction. Time-domain experiments illustrate the resulting estimation bounds.