In this paper, a fractional order HIV/HTLV co-infection model with HIV-specific antibody immune response is established. Two cases are considered: constant control and optimal control. For the constant control system, the existence and uniqueness of the positive solutions are proved, and then the sufficient conditions for the existence and stability of five equilibriums are obtained. For the second case, the Pontryagin's Maximum Principle is used to analyze the optimal control, and the formula of the optimal solution are derived. After that, some numerical simulations are performed to validate the theoretical prediction. Numerical simulations indicate that in the case of HIV/HTLV co-infection, the concentration of $CD{4}^{+}T$ cells is no longer suitable as an effective reference data for understanding the development process of the disease. On the contrary, the number of HIV virus particles should be used as an important indicator for reference.

In this paper, a fractional order differential algebraic predator-prey system with Holling type functional response was presented. The dynamic behavior of populations under the influence of economic benefits was thoroughly investigated. Specifically, with economic benefit as the bifurcation parameter, the existence of singularity-induced bifurcation was analyzed based on the theory of differential algebraic system. After that, a state feedback controller was designed to eliminate the singularity induced bifurcation and stabilize the system near the corresponding interior equilibrium. Furthermore, an optimal control problem was formulated, and the necessary conditions were derived. In addition, the validity of the theoretical results was confirmed through extensive numerical simulations.