This paper proposes an optimal control problem, with the final goal of implementing an optimal treatment protocol which could maximize the uninfected CD4+T cells and minimize the cost of drug, utilizing a system of ordinary differential equations which describes the interaction of the immune system with the human immunodeficiency virus(HIV). The optimal pair of control and trajectories of this nonlinear system with quadratic cost functional is obtained by Fourier series approximation. The method is based upon expanding time varying functions in the nonlinear system as their Fourier series, using the operational matrices for integration and product. The problem is reduced into a set of algebraic equations
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