Analogs of S.N. Bernstein and V.I. Smirnov inequalities for harmonic polynomials

S. Yu. Graf ; I. A. Nikitin

Vestnik Tverskogo gosudarstvennogo universiteta. Seriâ Prikladnaâ matematika, no. 2 (2021), pp. 16-25

Résumé

Harmonic mapings and, in particular, harmonic polynomials find applications in many problems of mathematics, mathematical physics, mechanics and electrical engineering. This is due to the key role that harmonic functions play in boundary value problems of mathematical physics. Harmonic polynomials are used to describe plane harmonic vector fields in hydrodynamics, in the theory of liquid crystals, in the theory of plane potential. Estimates of harmonic polynomials and their derivatives are used in the development of non-uniform grids and triangulations in many computational schemes. In the middle of the twentieth century, Soviet mathematicians S.N. Bernstein and V.I. Smirnov proved results several differential inequalities connecting the polynomials $P(z) = a_n z^n + a_{n-1} z^{n-1} + \dots a_1 z + a_0$ in the complex plane $\mathbb{C}$ and their derivatives. This topic remains important, as evidenced by the large number of new publications of Russian and foreign mathematicians. In this paper, we proved results that generalize the inequalities of S.N. Bernstein and V.I. Smirnov for the case of harmonic polynomials $F = H + \overline G,$ where $H, G$ are analytic polynomials. In particular, conditions of the type of majorizing inequalities on the unit circle are obtained, which make it possible to estimate the derivatives of the analytic and antianalytic parts of harmonic polynomials, all of whose zeros are located in the unit disk. The proofs of the main results are obtained using a topological analogue of the principle of the argument known in the theory of functions, which makes it possible to reduce some problems of the theory of harmonic polynomials to the analytic case. The classical inequalities of Smirnov and Bernstein in the case of analytic polynomials follow from the results of current paper. The proved theorems are illustrated by an example that demonstrates the accuracy of the conditions and estimates formulated by us.

Détail

Keywords: harmonic polynomials, Bernstein inequality, Smirnov inequality.