We study a problem of stochastic control in mathematical finance, with the goal of maximizing expected utility of investment and consumption over a finite trading horizon. The asset prices are modeled by Itô processes, for which the market parameters are subject to regime switching in the sense of being adapted to the joint filtration of the driving Brownian motion and a finite-state Markov chain which models "regime states" of the market. The vector of portfolios is constrained to a specified closed and convex set, and margin payments are levied on the investor, resulting in a wealth equation which is nonlinear in the portfolio. We proceed by the method of conjugate duality to construct a dual optimization problem together with optimality relations between putative solutions of the given (i.e., "primal") optimization problem and the dual optimization problem. These optimality relations are then used to address the specific cases of power-type and logarithmic utility functions, with convex cone portfolio constraints, and a higher rate of interest for borrowing than for lending. We get completely explicit optimal portfolios and characterize the optimal consumption rate as the solution of a backward stochastic differential equation (BSDE) "driven" by the canonical martingales of the regime-state Markov chain. For the power utility function this is a rather unconventional BSDE, to which standard existence results do not apply, and accordingly we establish existence and uniqueness of solutions for this BSDE.
Introduction.We address a problem of stochastic optimal control with the goal of trading over a finite time horizon to maximize expected utility from wealth at close of trade together with utility from intertemporal consumption. Our market model is quite classical, comprising a single risk-free asset and a finite number of risky assets, the prices of which are modeled by continuous-time Itô processes driven by a standard multidimensional Brownian motion. The problem includes (i) portfolio constraints, namely, the portfolio vector of holdings in the risky assets must take values in a specified closed convex set; (ii) margin payments levied on the investor, resulting in a wealth equation which is generally nonlinear in the portfolio; (iii) regime switching in the market model in the following sense: besides the standard Brownian motion driving the stock price models a further source of randomness in the model is a finite-state continuous-time Markov chain which is independent of the Brownian motion, it being stipulated that the market parameters (i.e., the risk-free interest rate, mean rate of return, and volatility of stocks) are adapted to the joint filtration of the Brownian motion and the Markov chain. Each state of the Markov chain models a market "mode" or "regime state"; a simple but apt example is that of a two-state