“…Robust stability and performance are achieved by minimizing an appropriate norm (such as · L 2 -L 2 (or · ∞ ), · L 2 -L∞ and · L∞-L∞ , which are called energy to energy, energy to peak, and peak to peak, respectively) of a transfer function. Considerable attention has been devoted to the optimal control problem, such as energy to energy (or H ∞ ) control and model reduction [4,6,25,26], H 2 (and mixed H 2 /H ∞ ) control and filtering [8,9,19,20,22,23], energy to peak (or L 2 -L ∞ ) control and filtering [7,21]. In many practical cases, the L 2 -L ∞ performance is more reasonable to achieve in system design.…”