This technical note revisits the open-loop Stackelberg strategy for a two-player game. By introducing a new costate, which captures the future information of the control input, we present a necessary and sufficient condition for the existence and uniqueness of the two-player game. The optimal strategy is designed in terms of three decoupled and symmetric Riccati equations which improves the existing results greatly on computation.Index Terms-Leader-follower, necessary and sufficient condition, Riccati equation, Stackelberg strategy.
I. INTRODUCTIONGame theory is of great significance in numerous fields such as economic, social science, political science and biology. Considerable attention has been devoted to the differential/difference game in [1], [3]-[5], [10], [21] and references therein. In general, the game is divided into zero-sum game [4] and nonzero-sum game [21] based on the difference of the performance criterions. According to the roles of the players, the strategies can be classified into Nash, minmax, noninferior, Stackelberg strategy, etc. [1] and [3] study the necessary conditions for the Nash game in terms of two coupled and nonsymmetric Riccati equations. The strategies of minmax and noninferior set are discussed in [21] by the value function approach. For the Stackelberg game, [18] presents an implicit necessary and sufficient condition for the unique existence of the solution based on the operator technique. It is shown that the positive definiteness of the weighting matrices in the performance is sufficient to ensure the uniqueness of the Stackelberg strategy. With the convex condition, [6] studies the properties of the solution to the corresponding algebraic Riccati equations. Reference [8] proposes a new sufficient existence condition for the open-loop equilibrium by adopting a Lyapunov-type approach. Both the finite-horizon and infinite-horizon linear-quadratic Stackelberg game including time preference rates are investigated in [11] via the matrix block technique. It should be noted that the aforementioned results are based on solving three coupled and nonsymmetric Riccati equations, under the stronger assumption that the weighting matrices of the cost functions are positive-definite.In this technical note, we revisit the Stackelberg strategy for a twoplayer game in discrete-time dynamic setting under a mild assumption. Our main contribution is the introduction of an appropriate costate which is defined as a vector capturing the effects of the future inputs.