Discovered in the seventies, Black-Scholes formula continues to play a central role in Mathematical Finance. We recall this formula. Let (B, t? 0; F, t? 0, P) - t t note a standard Brownian motion with B = 0, (F, t? 0) being its natural ?ltra- 0 t t tion. Let E: = exp B?, t? 0 denote the exponential martingale associated t t 2 to (B, t? 0). This martingale, also called geometric Brownian motion, is a model t to describe the evolution of prices of...