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Fedor Pakovich

Publications and source records attributed to Fedor Pakovich.

At least 37 records · Page 2Linked to original sources
arXiv2022

Tame rational functions: Decompositions of iterates and orbit intersections

Let $A$ be a rational function of degree at least two on the Riemann sphere. We say that $A$ is tame if the algebraic curve $A(x)-A(y)=0$ has no factors of genus zero or one distinct from the diagonal. In this paper, we show that if tame rational functions $A$ and $B$ have orbits with infinite intersection, then $A$ and $B$ have a common iterate. We also show that for a tame rational function $A$ decompositions of its iterates $A^{\circ d},$ $d\geq 1,$ into compositions of rational functions can be obtained from decompositions of a single iterate $A^{\circ N}$ for $N$ big enough.

math.DS↗
arXiv2022

Invariant curves for endomorphisms of $\mathbb P^1\times \mathbb P^1$

Let $A_1, A_2\in \mathbb C(z)$ be rational functions of degree at least two that are neither Lattès maps nor conjugate to $z^{\pm n}$ or $\pm T_n.$ We describe invariant, periodic, and preperiodic algebraic curves for endomorphisms of $(\mathbb P^1(\mathbb C))^2$ of the form $(z_1,z_2)\rightarrow (A_1(z_1),A_2(z_2)).$ In particular, we show that if $A\in \mathbb C(z)$ is not a "generalized Lattès map", then any $(A,A)$-invariant curve has genus zero and can be parametrized by rational functions commuting with $A$. As an application, for $A$ defined over a subfield $K$ of $ \mathbb C$ we give a criterion for a point of $(\mathbb P^1(K))^2$ to have a Zariski dense $(A, A)$-orbit in terms of canonical heights, and deduce from this criterion a version of a conjecture of Zhang on the existence of rational points with Zariski dense forward orbits. We also prove a result about functional decompositions of iterates of rational functions, which implies in particular that there exist at most finitely many $(A_1, A_2)$-invariant curves of any given bi-degree $(d_1,d_2).$

math.DS↗
arXiv2021

On amenable semigroups of rational functions

We characterize left and right amenable semigroups of polynomials of one complex variable with respect to the composition operation. We also prove a number of results about amenable semigroups of arbitrary rational functions. In particular, we show that under quite general conditions a semigroup of rational functions is left amenable if and only if it is a subsemigroup of the centralizer of some rational function.

math.DS↗
arXiv2020

Sharing a measure of maximal entropy in polynomial semigroups

Let $P_1,P_2,\dots, P_k$ be complex polynomials of degree at least two that are not simultaneously conjugate to monomials or to Chebyshev polynomials, and $S$ the semigroup under composition generated by $P_1,P_2,\dots, P_k$. We show that all elements of $S$ share a measure of maximal entropy if and only if the intersection of principal right ideals $SP_1\cap SP_2\cap \dots \cap SP_k$ is non-empty.

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arXiv2020

On rational functions sharing the measure of maximal entropy

We show that describing rational functions $f_1,$ $f_2,$ $\dots,f_n$ sharing the measure of maximal entropy reduces to describing solutions of the functional equation $A\circ X_1=A\circ X_2=\dots=A\circ X_n$ in rational functions. We also provide some results about solutions of this equation.

math.DS↗
arXiv2019

Commuting rational functions revisited

Let $B$ be a rational function of degree at least two that is neither a Lattès map nor conjugate to $z^{\pm n}$ or $\pm T_n$. We provide a method for describing the set $C_B$ consisting of all rational functions commuting with $B.$ Specifically, we define an equivalence relation $\underset{B}{\sim}$ on $ C_B$ such that the quotient $ C_B/\underset{B}{\sim}$ possesses the structure of a finite group $G_B$, and describe generators of $G_B$ in terms of the fundamental group of a special graph associated with $B$.

math.DS↗
arXiv2019

Algebraic curves $A^{\circ l}(x)-U(y)=0$ and arithmetic of orbits of rational functions

We give a description of pairs of complex rational functions $A$ and $U$ of degree at least two such that for every $d\geq 1$ the algebraic curve $A^{\circ d}(x)-U(y)=0$ has a factor of genus zero or one. In particular, we show that if $A$ is not a `generalized Lattès map', then this condition is satisfied if and only if there exists a rational function $V$ such that $U\circ V=A^{\circ l}$ for some $l\geq 1.$ We also prove a version of the dynamical Mordell-Lang conjecture, concerning intersections of orbits of points from $\mathbb P^1(K)$ under iterates of $A$ with the value set $U(\mathbb P^1(K))$, where $A$ and $U$ are rational functions defined over a number field $K.$

math.DS↗
arXiv2018

On generalized Lattès maps

We introduce a class of rational functions $A:\,\mathbb C\mathbb P^1\rightarrow \mathbb C\mathbb P^1$ which can be considered as a natural extension of the class of Lattès maps and establish basic properties of functions from this class.

math.DS↗
arXiv2018

Level curves of rational functions and unimodular points on rational curves

We obtain an improvement and broad generalisation of a result of N. Ailon and Z. Rudnick (2004) on common zeros of shifted powers of polynomials. Our approach is based on reducing this question to a more general question of counting intersections of level curves of complex functions. We treat this question via classical tools of complex analysis and algebraic geometry.

math.NT↗
arXiv2017

On algebraic curves A(x)-B(y)=0 of genus zero

Using a geometric approach involving Riemann surface orbifolds, we provide lower bounds for the genus of an irreducible algebraic curve of the form $E_{A,B}:\, A(x)-B(y)=0$, where $A, B\in\mathbb C(z)$. We also investigate "series" of curves $E_{A,B}$ of genus zero, where by a series we mean a family with the "same" $A$. We show that for a given rational function $A$ a sequence of rational functions $B_i$, such that ${\rm deg}\, B_i \rightarrow \infty$ and all the curves $A(x)-B_i(y)=0$ are irreducible and have genus zero, exists if and only if the Galois closure of the field extension $\mathbb C(z)/\mathbb C(A)$ has genus zero or one.

math.NT↗
arXiv2017

Recomposing rational functions

Let $A$ be a rational function. For any decomposition of $A$ into a composition of rational functions $A=U\circ V$ the rational function $\widetilde A=V\circ U$ is called an elementary transformation of $A$, and rational functions $A$ and $B$ are called equivalent if there exists a chain of elementary transformations between $A$ and $B$. This equivalence relation naturally appears in the complex dynamics as a part of the problem of describing of semiconjugate rational functions. In this paper we show that for a rational function $A$ its equivalence class $[A]$ contains infinitely many conjugacy classes if and only if $A$ is a flexible Lattès map. For flexible Lattès maps $L=L_j$ induced by the multiplication by 2 on elliptic curves with given $j$-invariant we provide a very precise description of $[ L]$. Namely, we show that any rational function equivalent to $ L_j$ necessarily has the form $ L_{j'}$ for some $j'\in \mathbb C$, and that the set of $j'\in \mathbb C$ such that $ L_{j'}\sim L_{j}$ coincides with the orbit of $j$ under the correspondence associated with the classical modular equation $Φ_2(x,y)=0$.

math.DS↗
arXiv2016

Polynomial semiconjugacies, decompositions of iterations, and invariant curves

We study the functional equation $A\circ X=X\circ B$, where $A,$ $B$, and $X$ are polynomials over $\mathbb C$. Using previous results of the author about polynomials sharing preimages of compact sets, we show that for given $B$ its solutions may be described in terms of the filled-in Julia set of $B$. On this base, we prove a number of results describing a general structure of solutions. The results obtained imply in particular the result of Medvedev and Scanlon about invariant curves of maps $F:\,\mathbb C^2 \rightarrow \mathbb C^2$ of the form $(x,y)\rightarrow (f(x),f(y))$, where $f$ is a polynomial, and a version of the result of Zieve and Müller about decompositions of iterations of a polynomial.

math.NT↗
arXiv2015

Minimum Degree of the Difference of Two Polynomials over $\mathbb Q$. Part II: Davenport-Zannier pairs

In this paper we study pairs of polynomials with a given factorization pattern and such that the degree of their difference attains its minimum. We call such pairs of polynomials Davenport--Zannier pairs, or DZ-pairs for short. The paper is devoted to the study of DZ-pairs with rational coefficients. In our earlier paper, in the framework of the theory of dessins d'enfants, we established a correspondence between DZ-pairs and weighted bicolored plane trees. These are bicolored plane trees whose edges are endowed with positive integral weights. When such a tree is uniquely determined by the set of black and white degrees of its vertices, it is called unitree, and the corresponding DZ-pair is defined over $\mathbb Q$. In our earlier paper, we classified all unitrees. In this paper, we compute all the corresponding polynomials. In the final part of the paper we present some additional material concerning the Galois theory of DZ-pairs and weighted trees.

math.NT↗
arXiv2014

Solution of the parametric center problem for the Abel differential equation

The Abel differential equation $y'=p(x)y^2+q(x)y^3$ with $p,q\in \mathbb R[x]$ is said to have a center on a segment $[a,b]$ if all its solutions, with the initial value $y(a)$ small enough, satisfy the condition $y(b)=y(a)$. The problem of description of conditions implying that the Abel equation has a center may be interpreted as a simplified version of the classical Center-Focus problem of Poincaré. The Abel equation is said to have a "parametric center" if for each $\varepsilon \in \mathbb R$ the equation $y'=p(x)y^2+\varepsilon q(x)y^3$ has a center. In this paper we show that the Abel equation has a parametric center if and only if the antiderivatives $P=\int p(x) dx,$ $Q=\int q(x) dx$ satisfy the equalities $P=\widetilde P \circ W,\ $ $Q=\widetilde Q\circ W$ for some polynomials $\widetilde P,$ $\widetilde Q,$ and $W$ such that $W(a)=W(b)$. We also show that the last condition is necessary and sufficient for the "generalized moments" $\int_a^b P^id Q$ and $\int_a^b Q^id P$ to vanish for all $i\geq 0.$

math.CA↗
arXiv2006

Conservative polynomials and yet another action of $\Gal(\bar \Q/\Q)$ on plane trees

In this paper we study an action of the absolute Galois group $Γ$ on bicolored plane trees induced by the action of $Γ$ on equivalence classes of conservative polynomials which are the simplest example of postcritically finite rational functions. We establish some basic properties of this action and compare it with the similar action provided by the Grothendieck theory of "Dessins d'enfants".

math.NT↗