Perpetual American Options in a Diffusion Model with Piecewise-Linear Coefficients

We derive closed form solutions to the discounted optimal stopping problems related to the pricing of the perpetual American standard put and call options in a diffusion model with piecewise-linear coefficients. The method of proof is based on the reduction of the initial optimal stopping problems to the associated free-boundary problems and the subsequent martingale verification using a local time-space formula. We present explicit algorithms to determine the constant hitting thresholds for the underlying asset price process, which provide the optimal exercise boundaries for the options