Time-consistent portfolio selection with strictly monotone mean-variance preference

This paper is devoted to time-consistent control problems of portfolio selection with strictly monotone mean-variance preferences. These preferences are variational modifications of the conventional mean-variance preferences, and remain time-inconsistent as in mean-variance optimization problems. To tackle the time-inconsistency, we study the Nash equilibrium controls of both the open-loop type and the closed-loop type, and characterize them within a random parameter setting. The problem is reduced to solving a flow of forward-backward stochastic differential equations for open-loop equilibria, and to solving extended Hamilton-Jacobi-Bellman equations for closed-loop equilibria. In particular, we derive semi-closed-form solutions for these two types of equilibria under a deterministic parameter setting. Both solutions are represented by the same function, which is independent of wealth state and random path. This function can be expressed as the conventional time-consistent mean-variance portfolio strategy multiplied by a factor greater than one. Furthermore, we find that the state-independent closed-loop Nash equilibrium control is a strong equilibrium strategy in a constant parameter setting only when the interest rate is sufficiently large.