Closed-form approximation of American options

Abstract

This paper presents a closed-form approximation of the value of the finite-lived American option where the underlying asset provides a constant pay-out rate. It is derived by imposing a restriction on the set of feasible exercise strategies, and thus represents a lower bound to the option value. The exercise strategy is to exercise the option the first time the price of the underlying asset hits a flat boundary. Our numerical results show that this lower bound is very close to the true option value.