20.9 The vf Model (page 460) The impied volati lity function model is designed to create a process for the asset price that exactly matches observed option prices. The usual model ds=(r-gSat +osdz is replaced b ds=[r(t)-g(t]dt +o(s, t)Saz Options, Futures, and other Derivatives, 5th edition 2002 by John C. HullOptions, Futures, and Other Derivatives, 5th edition © 2002 by John C. Hull 20.9 The IVF Model (page 460) dS r t q t dt S t Sdz dS r q Sdt Sdz [ ( ) ( )] ( , ) ( ) = − +  = − +  i s replaced by prices.The usual model price that exactly matches observed option designed to create a process for the asset The impied volatility function model i s