law in the pricing of derivatives. 22. Pfiska. S. R. fntroduction to Mathematical Finance: Discrete Time Modals (Blackwell. 1997). 23. Arrow, KJ. & Debreu. G. Existence of an equilibrium for a competitive economy. £conometrica 22, 265-290 (1954). 24. May, R. M. Stability and Complexity in Model Ecosys