. "For example, one possible formulation may be based on the exact solution of Rorden in which exponential decay is observed for large values of r/??, the equations may be expressed in the form: S i = S 0 ??? e - r ?? i ??? G ??? ( r ?? i ) ( 11 ) S i = e - c ??? ??? f i 0. ??? ??? 5 ??? ( S 0 + a ??? ??? f i 0.5 + b ??? ??? f i ) ( 12 ) where G is a function to be determined and all other values are" . . .