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The Black–Scholes model assumes positive underlying prices; if the underlying has a negative price, the model does not work directly. [ 51 ] [ 52 ] When dealing with options whose underlying can go negative, practitioners may use a different model such as the Bachelier model [ 52 ] [ 53 ] or simply add a constant offset to the prices.
The Greeks of European options (calls and puts) under the Black–Scholes model are calculated as follows, where (phi) is the standard normal probability density function and is the standard normal cumulative distribution function. Note that the gamma and vega formulas are the same for calls and puts.
Black and Scholes' insight was that the portfolio represented by the right-hand side is riskless: thus the equation says that the riskless return over any infinitesimal time interval can be expressed as the sum of theta and a term incorporating gamma.
As in the Black–Scholes model for stock options and the Black model for certain interest rate options, the value of a European option on an FX rate is typically calculated by assuming that the rate follows a log-normal process. [3] The earliest currency options pricing model was published by Biger and Hull, (Financial Management, spring 1983).
The approach arises since the evolution of the option value can be modelled via a partial differential equation (PDE), as a function of (at least) time and price of underlying; see for example the Black–Scholes PDE. Once in this form, a finite difference model can be derived, and the valuation obtained. [2]
Stochastic volatility models are one approach to resolve a shortcoming of the Black–Scholes model. In particular, models based on Black-Scholes assume that the underlying volatility is constant over the life of the derivative, and unaffected by the changes in the price level of the underlying security.
The model can be obtained with a change of variable from a standard Black-Scholes model as follows. By setting Y t = S t − β e r t {\displaystyle Y_{t}=S_{t}-\beta e^{rt}} it is immediate to see that Y follows a standard Black-Scholes model
If we observe = this model becomes a geometric Brownian motion as in the Black-Scholes model, whereas if = and either = or the drift is replaced by , this model becomes an arithmetic Brownian motion, the model which was proposed by Louis Bachelier in his PhD Thesis "The Theory of Speculation", known as Bachelier model.