Pricing Dynamic Investment Fund Protection

We consider an investment fund whose unit value is modeled by a geometric Brownian motion. Different forms of dynamic investment fund protection are examined. The basic form is a guarantee that instantaneously provides the necessary payments so that the upgraded fund unit value does not fall below a protected level. A closed form expression for the price of such a guarantee is derived. This result can also be applied to price a guarantee where the protected level is an exponential function of time. Moreover, it is explicitly shown how the protection can be generated by construction of the replicating portfolio. The dynamic investment fund protection is compared with the corresponding put option, and it is shown that for short time intervals the ratio of the prices approaches 2. Finally, a more exotic guarantee is considered, where the protected level is a given percentage of the maximal observed fund unit value. Assuming that the protected level remains constant once the payments have started, we obtain a surprisingly simple formula for the price of a perpetual guarantee. Several numerical and graphical illustrations show how the theoretical results can be implemented in practice.