Chebyshev's method for exponential maps

Abstract

It is proved that the Chebyshev's method applied to an entire function $f$ is a rational map if and only if $f(z)=p(z)e^{q(z)}$, for some polynomials $p$ and $q$. These are referred to as rational Chebyshev maps, and their fixed points are discussed in this article. It is seen that $\infty$ is a parabolic fixed point with multiplicity one bigger than the degree of $q$. Considering $q(z)=p(z{)}^n+c$, where $p$ is a linear polynomial, $n\in \mathbb{N}$ and $c$ is a non-zero constant, we show that the Chebyshev's method applied to $p{e}^q$ is affine conjugate to that applied to $z{e}^{z^n}$. We denote this by $C_n$. All the finite extraneous fixed points of $C_n$ are shown to be repelling. The Julia set $\mathcal{J}(C_n)$ of $C_n$ is found to be preserved under rotations of order $n$ about the origin. For each $n$, the immediate basin of $0$ is proved to be simply connected. For all $n\le 16$, we prove that $\mathcal{J}(C_n)$ is connected. For $n$ even, the non-existence of Herman ring and Siegel disk of $C_n$ is proved. Under some additional hypothesis, the same is also proved for odd $n$. The Newton's method applied to $z{e}^{z^n}$ is found to be conjugate to a polynomial, and its dynamics is also completely determined.

28 pages, 10 figures