A comparative analysis of Bernstein type estimates for the derivative of multivariate polynomials

Abstract

We compare the yields of two methods to obtain Bernstein type pointwise estimates for the derivative of a multivariate polynomial in points of some domain, where the polynomial is assumed to have sup norm at most 1. One method, due to Sarantopoulos, relies on inscribing ellipses into the convex domain K. The other, pluripotential theoretic approach, mainly due to Baran, works for even more general sets, and yields estimates through the use of the pluricomplex Green function (the Zaharjuta -Siciak extremal function). Using the inscribed ellipse method on non-symmetric convex domains, a key role was played by the generalized Minkowski functional a(K,x). With the aid of this functional, our current knowledge is precise within a constant (squareroot 2) factor. Recently L. Milev and the author derived the exact yield of this method in the case of the simplex, and a number of numerical improvements were obtained compared to the general estimates known. Here we compare the yields of this real, geometric method and the results of the complex, pluripotential theoretical approaches on the case of the simplex. In conclusion we can observe a few remarkable facts, comment on the existing conjectures, and formulate a number of new hypothesis.