“…Our previous method also offered suboptimal controllers [1], which may be regarded as an extension of gradient flow approaches [31], [32]. Unfortunately, these methods [1], [25], [26], [27], [28], [29], [31], [32] adopted only simple quadratic cost functions as performance metrics, the variety of which is limited. Innovative approaches with Kronecker products [33], [34] and the sum of squares [35], [36], [37], [38] can treat polynomial cost functions, which are less restrictive than quadratic functions.…”