“…We also assume that the following condition holds [8]: b 2 1 + b 2 2 ≤ 4(1 − γ)ac for some γ ∈ (0, 1). In recent years, numerous numerical methods have been widely applied to various optimal control problems governed by partial differential equations; see, e.g., [5,21,22] for standard finite element methods, [3,16,17] for mixed finite element methods, [11,18] for finite volume methods, and [6,9] for spectral methods. Chen and Liu [2] first used Raviart-Thomas mixed finite element method to solve a class of elliptic optimal control problems, in which objective functional contains the gradient of the state variable.…”