Density of multivariate homogeneous polynomials on star like domains

Kroó, András (2019) Density of multivariate homogeneous polynomials on star like domains. JOURNAL OF MATHEMATICAL ANALYSIS AND APPLICATIONS, 469 (1). pp. 239-251. ISSN 0022-247X

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Abstract

The famous Weierstrass theorem asserts that every continuous function on a compact set in R-d can be uniformly approximated by algebraic polynomials. A related interesting problem consists in studying the same question for the important subclass of homogeneous polynomials containing only monomials of the same degree. The corresponding conjecture claims that every continuous function on the boundary of convex 0-symmetric bodies can be uniformly approximated by pairs of homogeneous polynomials. The main objective of the present paper is to review the recent progress on this conjecture and provide a new unified treatment of the same problem on non convex star like domains. It will be shown that the boundary of every 0-symmetric non convex star like domain contains an exceptional zero set so that a continuous function can be uniformly approximated on the boundary of the domain by a sum of two homogeneous polynomials if and only if the function vanishes on this zero set. Thus the Weierstrass type approximation problem for homogeneous polynomials on non convex star like domains amounts to the study of these exceptional zero sets. We will also present an extension of a theorem of Varju which describes the exceptional zero sets for intersections of star like domains. These results combined with certain transformations of the underlying region will lead to the discovery of some new classes of convex and non convex domains for which the Weierstrass type approximation result holds for homogeneous polynomials. (C) 2018 Elsevier Inc. All rights reserved.

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