sentences:  Consider a classical Black-Scholes set-up and let formula_72 be the risk-free interest rate and formula_73 and formula_74 be the dividend rate and volatility of the stock.The stock price formula_75 follows geometric Brownian motion
  under the risk-neutral measure.When the option is perpetual, the optimal stopping problem is
  where the payoff function is formula_78 for a call option and formula_79 for a put option.