INEXACT OBJECTIVE FUNCTION EVALUATIONS IN A TRUST-REGION ALGORITHM FOR PDE-CONSTRAINED OPTIMIZATION UNDER UNCERTAINTY

Abstract. This paper improves the trust-region algorithm with adaptive sparse grids introduced in [?] for the solution of optimization problems governed by partial differential equations (PDEs) with uncertain coefficients. The previous algorithm used adaptive sparse grid discretizations to generate models that are applied in a trust-region framework to generate a trial step. The decision whether to accept this trial step as the new iterate, however, required relatively high fidelity adaptive discretizations of the objective function. In this paper, we extend the algorithm and convergence theory to allow the use of low-fidelity adaptive sparse-grid models in objective function evaluations. This is accomplished by extending conditions on inexact function evaluations used in previous trust-region frameworks. Our algorithm adaptively builds two separate sparse grids: one to generate optimization models for the step computation, and one to approximate the objective function. These adapted sparse grids typically contain significantly fewer points than the high-fidelity grids, which leads to a dramatic reduction in the computational cost. This is demonstrated numerically using two examples. Moreover, the numerical results indicate that the new algorithm rapidly identifies the stochastic variables that are relevant to obtaining an accurate optimal solution. When the number of such variables is independent of the dimension of the stochastic space, the algorithm exhibits near dimension-independent behavior. Key words. PDE-constrained optimization, uncertainty, stochastic collocation, trust regions, sparse grids, adaptivity AMS subject classifications. 49M15, 65K05, 65N35, 90C15 1. Introduction. Th