On the valence of logharmonic polynomials

Author:

Khavinson Dmitry,Lundberg Erik,Perry Sean

Abstract

Investigating a problem posed by W. Hengartner (2000), we study the maximal valence (number of preimages of a prescribed point in the complex plane) of logharmonic polynomials, i.e., complex functions that take the form f ( z ) = p ( z ) q ( z ) ¯ f(z) = p(z) \overline {q(z)} of a product of an analytic polynomial p ( z ) p(z) of degree n n and the complex conjugate of another analytic polynomial q ( z ) q(z) of degree m m . In the case m = 1 m=1 , we adapt an indirect technique utilizing anti-holomorphic dynamics to show that the valence is at most 3 n 1 3n-1 . This confirms a conjecture of Bshouty and Hengartner (2000). Using a purely algebraic method based on Sylvester resultants, we also prove a general upper bound for the valence showing that for each n , m 1 n,m \geq 1 the valence is at most n 2 + m 2 n^2+m^2 . This improves, for every choice of n , m 1 n,m \geq 1 , the previously established upper bound ( n + m ) 2 (n+m)^2 based on Bezout’s theorem. We also consider the more general setting of polyanalytic polynomials where we show that this latter result can be extended under a nondegeneracy assumption.

Publisher

American Mathematical Society

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