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 ??? ??? - r ?? i ??? G ??? ( r ?? i ) ( 11 ) S i = ??? - cf i 0.5 ??? ( S 0 + af i 0.5 + bf i ) ( 12 ) where G is a function to be determined and all other values are described above.