  State-space formulas are derived for the minimum-entropy $\mathcal{H}_\infty$
controller when the plant and controller are constrained to be
block-lower-triangular. Such a controller exists if and only if: the
corresponding unstructured problem has a solution, a certain pair of coupled
algebraic Riccati equations admits a mutually stabilizing fixed point, and a
pair of spectral radius conditions is met. The controller's observer-based
structure is also discussed, and a simple numerical approach for solving the
coupled Riccati equations is presented.
