2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/208906We consider the problem of finding the optimal time to sell a stock, subject to a fixed sales cost and an exponential discounting rate ρ. We assume that the price of the stock fluctuates according to the equation dY_t=Y_t(μdt+σξ(t) dt), where (ξ(t)) is an alternating Markov renewal process with values in {\pm1}, with an exponential renewal time. We determine the critical value of ρunder which the value function is finite. We examine the validity of the ``principle of smooth fit'' and use this to give a complete and essentially explicit solution to the problem, which exhibits a surprisingly rich structure. The corresponding result when the stock price evolves according to the Black and Scholes model is obtained as a limit case.Published at http://dx.doi.org/10.1214/105051604000000747 in the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org)ProbabilityPricing of Securities60G40 (Primary) 90A09\sep60J27 (Secondary)text