The optimality of a dividend barrier strategy for Levy insurance risk processes, with a focus on the univariate Erlang mixture
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In insurance risk theory, the surplus of an insurance company is modelled to monitor and quantify its risks. With the outgo of claims and inﬂow of premiums, the insurer needs to determine what ﬁnancial portfolio ensures the soundness of the company’s future while satisfying the shareholders’ interests. It is usually assumed that the net proﬁt condition (i.e. the expectation of the process is positive) is satisﬁed, which then implies that this process would drift towards inﬁnity. To correct this unrealistic behaviour, the surplus process was modiﬁed to include the payout of dividends until the time of ruin. Under this more realistic surplus process, a topic of growing interest is determining which dividend strategy is optimal, where optimality is in the sense of maximizing the expected present value of dividend payments. This problem dates back to the work of Bruno De Finetti (1957) where it was shown that if the surplus process is modelled as a random walk with ± 1 step sizes, the optimal dividend payment strategy is a barrier strategy. Such a strategy pays as dividends any excess of the surplus above some threshold. Since then, other examples where a barrier strategy is optimal include the Brownian motion model (Gerber and Shiu (2004)) and the compound Poisson process model with exponential claims (Gerber and Shiu (2006)). In this thesis, we focus on the optimality of a barrier strategy in the more general Lévy risk models. The risk process will be formulated as a spectrally negative Lévy process, a continuous-time stochastic process with stationary increments which provides an extension of the classical Cramér-Lundberg model. This includes the Brownian and the compound Poisson risk processes as special cases. In this setting, results are expressed in terms of “scale functions”, a family of functions known only through their Laplace transform. In Loeffen (2008), we can ﬁnd a sufﬁcient condition on the jump distribution of the process for a barrier strategy to be optimal. This condition was then improved upon by Loeffen and Renaud (2010) while considering a more general control problem. The ﬁrst chapter provides a brief review of theory of spectrally negative Lévy processes and scale functions. In chapter 2, we deﬁne the optimal dividends problem and provide existing results in the literature. When the surplus process is given by the Cramér-Lundberg process with a Brownian motion component, we provide a sufﬁcient condition on the parameters of this process for the optimality of a dividend barrier strategy. Chapter 3 focuses on the case when the claims distribution is given by a univariate mixture of Erlang distributions with a common scale parameter. Analytical results for the Value-at-Risk and Tail-Value-at-Risk, and the Euler risk contribution to the Conditional Tail Expectation are provided. Additionally, we give some results for the scale function and the optimal dividends problem. In the ﬁnal chapter, we propose an expectation maximization (EM) algorithm similar to that in Lee and Lin (2009) for ﬁtting the univariate distribution to data. This algorithm is implemented and numerical results on the goodness of ﬁt to sample data and on the optimal dividends problem are presented.