Paper 3, Section II, I

Stochastic Financial Models
Part II, 2010

Consider a market with two assets, a riskless bond and a risky stock, both of whose initial (time-0) prices are B0=1=S0B_{0}=1=S_{0}. At time 1 , the price of the bond is a constant B1=R>0B_{1}=R>0 and the price of the stock S1S_{1} is uniformly distributed on the interval [0,C][0, C] where C>RC>R is a constant.

Describe the set of state price densities.

Consider a contingent claim whose payout at time 1 is given by S12S_{1}^{2}. Use the fundamental theorem of asset pricing to show that, if there is no arbitrage, the initial price of the claim is larger than RR and smaller than CC.

Now consider an investor with initial wealth X0=1X_{0}=1, and assume C=3RC=3 R. The investor's goal is to maximize his expected utility of time-1 wealth EU[R+π(S1R)]\mathbb{E} U\left[R+\pi\left(S_{1}-R\right)\right], where U(x)=xU(x)=\sqrt{x}. Show that the optimal number of shares of stock to hold is π=1\pi^{*}=1.

What would be the investor's marginal utility price of the contingent claim described above?