Where E represents the expected value in the risk-centered world, T option expiration time, time after t, ST stock price at time T in the future, stock price at time after X, O square is the unit time stock The variance of the logarithmic change in price, a partial differential equation with an analytical solution. Finance students learn partial differential equations mainly because the Black-Scholes-Merton formula for option pricing is derived using partial differential equations (of course, risk neutrality is also commonly used nowadays. Equivalent martingale method)