  This paper deals with numerical solutions to an impulse control problem
arising from optimal portfolio liquidation with bid-ask spread and market price
impact penalizing speedy execution trades. The corresponding dynamic
programming (DP) equation is a quasi-variational inequality (QVI) with solvency
constraint satisfied by the value function in the sense of constrained
viscosity solutions. By taking advantage of the lag variable tracking the time
interval between trades, we can provide an explicit backward numerical scheme
for the time discretization of the DPQVI. The convergence of this discrete-time
scheme is shown by viscosity solutions arguments. An optimal quantization
method is used for computing the (conditional) expectations arising in this
scheme. Numerical results are presented by examining the behaviour of optimal
liquidation strategies, and comparative performance analysis with respect to
some benchmark execution strategies. We also illustrate our optimal liquidation
algorithm on real data, and observe various interesting patterns of order
execution strategies. Finally, we provide some numerical tests of sensitivity
with respect to the bid/ask spread and market impact parameters.
