“…Their work drew attention to [26], and soon afterwards many leading nonlinear control researchers starting using high-gain observers; c.f. [11], [20], [32], [34], [36], [37], [38], [39], [40], [56], [57] [58], [60], [61], [63], [78], [80], [83], [87], [88], [90], [97]. These papers have studied a wide range of nonlinear control problems, including stabilization, regulation, tracking, and adaptive control.…”