On s-convex approximations

Let B-s([a, b]; mu (1), mu (2),...,mu (s-1)) be the class of all distribution functions of random variables with support in [a, b] having mu (1), mu (2),..., mu (s-1) as their first s - 1 moments. In this paper we examine some aspects of the structure of B-s ([a, b]; mu (1), mu (2),..., mu (s-1)) and of the s -convex stochastic extrema in it. Using representation results of moment matrices Li la Lindsay (1989a), we provide conditions for the admissibility of moment sequences in B-s ([a, b]; mu (1), mu (2),..., mu (s-1)) in terms of lower bounds on the number of support points of the corresponding distribution functions. We point out two special distributions with a minimal number of support points that are the s-convex extremal distributions. It is shown that the support points of these extrema are the roots of some polynomials, and an efficient method for the complete determination of the distribution functions of these extrema is described. A study of the goodness of fit, of the approximation of an arbitrary element in B-s ([a, b]; mu (1), mu (2),..., mu (s-1)) by One of the stochastic s-convex extrema, is then given. Using standard ideas from linear regression, we derive Tchebycheff-type inequalities which extend previous results of Lindsay (1989b), and we establish some limit theorems involving the moment matrices. Finally, we describe some applications in insurance theory, namely, we provide bounds on Lundberg's coefficient in risk theory, and on the actual interest rate relating to a life insurance policy. These bounds are obtained with the aid of the s-convex extrema, and are determined only by the support and the first few moments of the underlying distribution.