7. Conclusion
This paper studies a continuous time mean-variance portfolio selection problem when the financial market consists of only risky assets whose price processes are modelled by Markov-modulated geometric Brownian motions. By introducing the Lagrange multiplier, the efficient frontier and efficient portfolio are expressed in closed form via three systems of ordinary differential equations. The global minimum variance is obtained, and the mutual fund theorem is proved by the fact that the efficient feedback portfolio is an affine function of the expected wealth level at the terminal time T. A few extensions can be made in our future research. The asset liability management problem could be investigated by taking into account the investor's endogenous or exogenous liability process. Additionally, to reflect limitations in the real market, constraints, such as prohibitions of short-selling the risky assets, may be imposed.