We consider an insurance company whose surplus is represented by the classical Cramer-Lundberg process. The company can invest its surplus in a risk-free asset and in a risky asset, governed by the Black-Scholes equation. There is a constraint that the insurance company can only invest in the risky asset at a limited leveraging level; more precisely, when purchasing, the ratio of the investment amount in the risky asset to the surplus level is no more than a; and when short-selling, the proportion of the proceeds from the short-selling to the surplus level is no more than b. The objective is to find an optimal investment policy that minimizes the probability of ruin. The minimal ruin probability as a function of the initial surplus is characterized by a classical solution to the corresponding Hamilton- Jacobi-Bellman (HJB) equation. We study the optimal control policy and its properties. The interrelation between the parameters of the model plays a crucial role in the qualitative behavior of the optimal policy. For example, for some ratios between a and b, quite unusual and at first ostensibly counterintuitive policies may appear, like short-selling a stock with a higher rate of return to earn lower interest, or borrowing at a higher rate to invest in a stock with lower rate of return. This is in sharp contrast with the unrestricted case, first studied in Hipp and Plum, or with the case of no shortselling and no borrowing studied in Azcue and Muler.
In this paper we consider the problem of finding an optimal excess of loss reinsurance which maximizes the reliability (probability of no ruin) of the insurance company. We apply two approximate approaches to calculate the distribution of total payments. The first approach is based on normal approximation of the payments distribution. Using this approximation we have derived an integral equation on the optimal retention limit. The second approach is based on simulation techniques. To test the precision of our approaches we use an exact formula for the distribution of total payments known for the case when losses in one insured event are distributed uniformly.