“…The HN filtering problem is to design an estimator to estimate the unknown state combination via output measurement, which guarantees that the L 2 -induced gain from the external disturbance to the estimation error is less than a prescribed level [1][2][3][4][5][6][7][8][9][10][11][12]. The HN filtering problem has been extensively studied over the past few decades and its applications in a variety of areas such as signal processing, signal estimation, pattern recognition and many practical control systems have been discussed.…”