Example 6.5.2 (Perpetual American put) Find the value of a perpetual American put option on non-dividend-paying stock, that is a contract that the holder can choose to exercise at any time t in which case the payoff is (K-S). Example 6.5.2 (Perpetual American put) Find the value of a perpetual American put option on non-dividend-paying stock, that is a contract that the holder can choose to exercise at any time t in which case the payoff is (K-S)