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Hiroki Sumi

Publications and source records attributed to Hiroki Sumi.

At least 19 recordsLinked to original sources

Random dynamical systems of polynomial automorphisms on $\Bbb{C}^{2}$

This paper deals with random dynamical systems of polynomial automorphisms (complex generalized Hénon maps and their conjugate maps) of $\Bbb{C}^{2}.$ We show that a generic random dynamical system of polynomial automorphisms has ``mean stablity'' on $\Bbb{C}^{2}$. Further, we show that if a system has mean stability, then (1) for each $z\in \Bbb{C}^{2}$ and for almost every sequence $γ=(γ_{n})_{n=1}^{\infty }$ of maps, the maximal Lyapunov exponents of $γ$ at $z$ is negative, (2) there are only finitely many minimal sets of the system, (3) each minimal set is attracting, (4) for each $z\in \Bbb{C}^{2}$ and for almost every sequence $γ$ of maps, the orbit $\{ γ_{n}\cdots γ_{1}(z) \} _{n=1}^{\infty }$ tends to one of the minimal sets of the system, and (5) the transition operator of the system has the spectrum gap property on the space of Hoelder continuous functions with some exponent. Note that none of (1)--(5) can hold for any deterministic iteration dynamical system of a single complex generalized Hénon map. We observe many new phenomena in random dynamical systems of polynomial automorphisms of $\Bbb{C}^{2}$ and observe the mechanisms. We provide new strategies and methods to study higher-dimensional random holomorphic dynamical systems.

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Thick-Thin non-Autonomous Julia Sets

Hereditarily non uniformly perfect (HNUP) sets were introduced by Stankewitz, Sugawa, and Sumi in \cite{SSS} who gave several examples of such sets based on Cantor set-like constructions using nested intervals. For non-autonomous iteration where one considers compositions of polynomials from a sequence which is in general allowed to vary, the Julia set is uniformly perfect for all sequences with suitably bounded coefficients, while Comerford, Stankewitz and Sumi showed in \cite{CSS} that for certain sequences of polynomials with unbounded coefficients, it is possible to have Julia sets which are HNUP. In this manuscript we give an example of a non-autonomous polynomial sequences whose Julia sets lie in between these two extremes in that they are not uniformly perfect, but also not HNUP. In addition we show that these Julia sets can be expressed as a `thick-thin' decomposition consisting of a ${\mathrm F}_σ$ subset which is a countable union of uniformly perfect sets and a ${\mathrm G}_δ$ subset which is HNUP.

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Packing measure and dimension of the limit sets of IFSs of generalized complex continued fractions

We consider a family of conformal iterated function systems (for short, CIFSs) of generalized complex continued fractions. Note that in our previous paper we showed that the proper-dimensional Hausdorff measure of the limit set is zero and the packing measure of the limit set with respect to the Hausdorff dimension is positive. In this paper, we show that the packing dimension and the Hausdorff dimension of the limit set of each CIFS in the family are equal, and the proper-dimensional packing measure of the limit set is finite.

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Bowen's formula for a rational graph-directed Markov system

We establish Bowen's formula for the Julia set of a non-elementary, expanding, irreducible and aperiodic rational graph-directed Markov system satisfying the backward separating condition. Towards this end, we shall prove that the associated skew product map is topologically exact on the skew product Julia set, and satisfies the density of repelling periodic points. Moreover, we give a criterion for expandingness in terms of hyperbolicity.

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The Dynamics and Geometry of Semi-Hyperbolic Rational Semigroups

We study skew-product dynamics for a large class of finitely-generated semi--hyperbolic semigroups of rational maps acting on the Riemann sphere, which generalizes both the theory of iteration of a single rational map of a single complex variable complex/holomorphic dynamics) and the theory of countable alphabet conformal iterated function systems (CIFSs). We construct the thermodynamic formalism for such dynamical systems and geometric potentials by developing the notion of nice families that extend to the case of our highly disconnected skew product phase space the powerful notion of nice sets due to Rivera--Letelier and Przytycki, and the allied earlier notion of $K(V)$ sets due to Denker and the last named author. We leverage out techniques to prove the existence and uniqueness of equilibrium states for a wide class of Hölder potentials, and concomitant statistical laws: central limit theorem, law of iterated logarithm, and exponential decay of correlations. We devote lots of space and effort to control (non-recurrent) critical points which is a notoriously challenging task even for a single rational function; more generators add qualitatively new challenges. Beyond dynamics, but still with dynamical methods, we advance the study of finer fractal geometrical properties of the intricate Julia sets associated to such systems and, in particular, via equilibrium states, we perform a multifractal analysis of Lyapunov exponents. We use the Nice Open Set Condition (NOSC) introduced by the last two authors, and apply our new techniques to settle a long-standing problem in the theory of rational semigroups by proving that for our class of semigroups the Hausdorff dimension of each fiber Julia set is strictly smaller than the Hausdorff dimension of the global Julia set of the semigroup.

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Non-i.i.d. random holomorphic dynamical systems and the generic dichotomy

We consider non-i.i.d. random holomorphic dynamical systems whose choice of maps depends on Markovian rules. We show that generically, such a system is mean stable or chaotic with full Julia set. If a system is mean stable, then the Lyapunov exponent is uniformly negative for every initial value and almost every random orbit. Moreover, we consider families of random holomorphic dynamical systems and show that the set of mean stable systems has full measure under certain conditions. The latter is a new result even for i.i.d. random dynamical systems.

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Negativity of Lyapunov Exponents and Convergence of Generic Random Polynomial Dynamical Systems and Random Relaxed Newton's Methods

We investigate i.i.d. random complex dynamical systems generated by probability measures on finite unions of the loci of holomorphic families of rational maps on the Riemann sphere. We show that under certain conditions on the families, for a generic system, (especially, for a generic random polynomial dynamical system,) for all but countably many initial values $z$ in the Riemann sphere, for almost every sequence of maps $γ=(γ_{1},γ_{2},\ldots )$, the Lyapunov exponent of $γ$ at $z$ is negative. Also, we show that for a generic system, for every initial value $z$ in the Riemann sphere, the orbit of the Dirac measure at $z$ under the iteration of the dual map of the transition operator tends to a periodic cycle of measures in the space of probability measures on the Riemann sphere. Note that these are new phenomena in random complex dynamics which cannot hold in deterministic complex dynamical systems. We apply the above theory and results of random complex dynamical systems to finding roots of any polynomials by random relaxed Newton's methods and we show that for any polynomial $g$, for any initial value $z$ in the complex plane which is not a root of $g'$, the random orbit starting with $z$ tends to a root of $g$ almost surely, which is the virtue of the effect of randomness.

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Uniformly perfect and hereditarily non uniformly perfect analytic and conformal non-autonomous attractor sets

Conditions are given which imply that certain non-autonomous analytic iterated function systems (NIFS's) in the complex plane have uniformly perfect attractor sets, while other conditions imply the attractor is pointwise thin, and thus hereditarily non uniformly perfect. Examples are given to illustrate the main theorems, as well as to indicate how they generalize other results. Examples are also given to illustrate how possible generalizations of corresponding results for autonomous IFS's do not hold in general in this more flexible setting. Further, applications to non-autonomous Julia sets are given. Lastly, since our definition of NIFS is in some ways more general than others found in the literature, a careful analysis is given to show when certain familiar relationships still hold, along with detailed examples showing when other relationships do not hold.

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Spectral gap property for random dynamics on the real line and multifractal analysis of generalised Takagi functions

We consider the random iteration of finitely many expanding $\mathcal{C}^{1+ε}$ diffeomorphisms on the real line without a common fixed point. We derive the spectral gap property of the associated transition operator acting on Hölder spaces. As an application we introduce generalised Takagi functions on the real line and we perform a complete multifractal analysis of the pointwise Hölder exponents of these functions.

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The Hausdorff dimension function of the family of conformal iterated function systems of generalized complex continued fractions

We consider the family of CIFSs of generalized complex continued fractions with a complex parameter space. This is a new interesting example to which we can apply a general theory of infinite CIFSs and analytic families of infinite CIFSs. We show that the Hausdorff dimension function of the family of the CIFSs of generalized complex continued fractions is continuous in the parameter space and is real-analytic and subharmonic in the interior of the parameter space. As a corollary of these results, we also show that the Hausdorff dimension function has a maximum point and the maximum point belongs to the boundary of the parameter space.

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Hausdorff measures and packing measures of the limit sets of CIFSs of generalized complex continued fractions

We consider a family of CIFSs of the generalized complex continued fractions with a complex parameter space. We show that for each CIFS of the family, the Hausdorff measure of the limit set of the CIFS with respect to the Hausdorff dimen/sion is zero and the packing measure of the limit set of the CIFS with respect to the Hausdorff dimension is positive (main result). This is a new phenomenon of infinite CIFSs which cannot hold in finite CIFSs. We prove the main result by showing some estimates for the unique conformal measure of each CIFS of the family and by using some geometric observations.

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Non-i.i.d. random holomorphic dynamical systems and the probability of tending to infinity

We consider random holomorphic dynamical systems on the Riemann sphere whose choices of maps are related to Markov chains. Our motivation is to generalize the facts which hold in i.i.d. random holomorphic dynamical systems. In particular, we focus on the function $T$ which represents the probability of tending to infinity. We show some sufficient conditions which make $T$ continuous on the whole space and we characterize the Julia sets in terms of the function $T$ under certain assumptions.

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Hereditarily non Uniformly Perfect non-Autonomous Julia Sets

Hereditarily non uniformly perfect (HNUP) sets were introduced by Stankewitz, Sugawa, and Sumi in \cite{SSS} who gave several examples of such sets based on Cantor set-like constructions using nested intervals. We exhibit a class of examples in non-autonomous iteration where one considers compositions of polynomials from a sequence which is in general allowed to vary. In particular, we give a sharp criterion for when Julia sets from our class will be HNUP and we show that the maximum possible Hausdorff dimension of $1$ for these Julia sets can be attained. The proof of the latter considers the Julia set as the limit set of a non-autonomous conformal iterated function system and we calculate the Hausdorff dimension using a version of Bowen's formula given in the paper by Rempe-Gillen and Urbánski \cite{RU}.

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Multifractal Formalism for generalised local dimension spectra of Gibbs measures on the real line

We refine the multifractal formalism for the local dimension of a Gibbs measure $μ$ supported on the attractor $Λ$ of a conformal iterated functions system on the real line. Namely, for given $α\in \mathbb{R}$, we establish the formalism for the Hausdorff dimension of level sets of points $x\inΛ$ for which the $μ$-measure of a ball of radius $r_{n}$ centered at $x$ obeys a power law $r_{n}{}^α$, for a sequence $r_{n}\rightarrow0$. This allows us to investigate the Hölder regularity of various fractal functions, such as distribution functions and conjugacy maps associated with conformal iterated function systems.

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Pointwise Hölder Exponents of the Complex Analogues of the Takagi Function in Random Complex Dynamics

We investigate the Hölder regularity of the function $T$ of the probability of tending to one minimal set, the partial derivatives of $T$ with respect to the probability parameters, which can be regarded as complex analogues of the Takagi function, and the higher partial derivatives $C$ of $T.$ Our main result gives a dynamical description of the pointwise Hölder exponents of $T$ and $C$, which allows us to determine the spectrum of pointwise Hölder exponents by employing the multifractal formalism in ergodic theory. Also, we prove that the bottom of the spectrum $α_{-}$ is strictly less than $1$, which allows us to show that the averaged system acts chaotically on the Banach space $C^{α}$ of $α$- Hölder continuous functions for every $α\in (α_{-},1)$, though the averaged system behaves very mildly (e.g. we have spectral gaps) on $C^{β}$ for small $β>0.$

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Dynamics of infinitely generated nicely expanding rational semigroups and the inducing method

We investigate the dynamics of semigroups of rational maps on the Riemann sphere. To establish a fractal theory of the Julia sets of infinitely generated semigroups of rational maps, we introduce a new class of semigroups which we call nicely expanding rational semigroups. More precisely, we prove Bowen's formula for the Hausdorff dimension of the pre-Julia sets, which we also introduce in this paper. We apply our results to the study of the Julia sets of non-hyperbolic rational semigroups. For these results, we do not assume the cone condition, which has been assumed in the study of infinite contracting iterated function systems. Similarly, we show that Bowen's formula holds for the limit set of a contracting conformal iterated function system without the cone condition.

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Hereditarily Non Uniformly Perfect Sets

We introduce the concept of hereditarily non uniformly perfect sets, compact sets for which no compact subset is uniformly perfect, and compare them with the following: Hausdorff dimension zero sets, logarithmic capacity zero sets, Lebesgue 2-dimensional measure zero sets, and porous sets. In particular, we give an example of a compact set in the plane of Hausdorff dimension 2 (and positive logarithmic capacity) which is hereditarily non uniformly perfect.

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Backward Iteration Algorithms for Julia sets of Möbius Semigroups

We extend a result regarding the Random Backward Iteration algorithm for drawing Julia sets (known to work for certain rational semigroups containing a non-Möbius element) to a class of Möbius semigroups which includes certain settings not yet been dealt with in the literature, namely, when the Julia set is not a thick attractor in the sense given in [David Fried, Sebastian. M. Marotta, and Rich Stankewitz, Complex dynamics of Möbius semigroups, Ergodic Theory Dynam. Systems, 32(6):1889--1929, 2012].

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