Algorithms for solving nonlinear dynamic decision models

Abstract

In this paper we discuss two Newton-type algorithms for solving economic models. The models are preprocessed by reordering the equations in order to minimize the dimension of the simultaneous block. The solution algorithms are then applied to this block. The algorithms evaluate numerically, as required, selected columns of the Jacobian of the simultaneous part. Provisions also exist for similar systems to be solved, if possible, without actually reinitialising the Jacobian. One of the algorithms also uses the Broyden update to improve the Jacobian. Global convergence is maintained by an Armijo-type stepsize strategy.

The global and local convergence of the quasi-Newton algorithm is discussed. A novel result is established for convergence under relaxed descent directions and relating the achievement of unit stepsizes to the accuracy of the Jacobian approximation. Furthermore, a simple derivation of the Dennis-Moré characterisation of the Q-superlinear convergence rate is given.

The model equation reordering algorithm is also described. The model is reordered to define heart and loop variables. This is also applied recursively to the subgraph formed by the loop variables to reduce the total number of above diagonal elements in the Jacobian of the complete system. The extension of the solution algorithms to consistent expectations are discussed. The algorithms are compared with Gauss-Seidel SOR algorithms using the USA and Spanish models of the OECD Interlink system.