Extremal Trigonometrical and Power Polynomials of Several Variables
  We consider the set of the power non-negative polynomials of several variables and its subset that consists of polynomials which can be represented as a sum of squares. It is shown in the classic work by D.Hilbert that it is a proper subset. Both sets are convex. In our paper we have made an attempt to work out a general approach to the investigation of the extremal elements of these convex sets. We also consider the class of non-negative rational functions. The article is based on the following methods: 1.We investigate non-negative trigonometrical polynomials and then with the help of the Calderon transformation we proceed to the power polynomials. 2.The way of constructing support hyperplanes to the convex sets is given in the paper. 
L.A. Sakhnovich