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Michael Yampolsky

Publications and source records attributed to Michael Yampolsky.

At least 55 records · Page 3Linked to original sources

The fixed point of the parabolic renormalization operator

We study parabolic renormalization of analytic germs with a simple parabolic point at the origin. We describe a class of maps $\mathbf P$ which admit a maximal analytic extension to a Jordan domain, and whose covering properties have an explicit topological model. We demonstrate that $\mathbf P$ is invariant under parabolic renormalization, and that Inou-Shishikura fixed point $f_*$ lies in $\mathbf P$. We conjecture that successive parabolic renormalizations of every map in $\mathbf P$ converge to $f_*$ at a geometric rate. We further present a numerical method for computing the Taylor's expansion of $f_*$ with a high accuracy. Our approach also allows us to compute the images of the maximal domain of analyticity of $f_*$. Finally, we obtain numerical estimates on the spectral radius of the differential of the parabolic renormalization operator at $f_*$.

math.DS↗

Abnormality of the placental vasculature affects placental thickness

Our empirical modeling suggests that deformation of placental vascular growth is associated with abnormal placental chorionic surface shape. Altered chorionic surface shape is associated with lowered placental functional efficiency. We hypothesize that placentas with deformed chorionic surface vascular trees and reduced functional efficiency also have irregular vascular arborization that will be reflected in increased variability of placental thickness and a lower mean thickness. We find that non-centrality of the umbilical cord insertion is strongly and significantly correlated with disk thickness (Spearman's rho=0.128, p=0.002). Deformed shape is strongly and significantly associated with lower overall thickness and higher variability of thickness with beta between -0.173 and -0.254 (p<0.001) . Both lower mean thickness and high variability of thickness are strongly correlated with higher beta (reduced placental efficiency) (p<0.001 and p=0.038 respectively). Greater thickness variability is correlated with higher beta independent of the other placental shape variables p=0.004.

q-bio.TO↗

Geometrization of postcritically finite branched coverings

We study canonical decompositions of postcritically finite branched coverings of the 2-sphere, as defined by K.~Pilgrim. We show that every hyperbolic cycle in the decomposition does not have a Thurston obstruction. It is thus Thurston equivalent to a rational map.

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Computability of Brolin-Lyubich Measure

Brolin-Lyubich measure $λ_R$ of a rational endomorphism $R:\riem\to\riem$ with $°R\geq 2$ is the unique invariant measure of maximal entropy $h_{λ_R}=h_{\text{top}}(R)=\log d$. Its support is the Julia set $J(R)$. We demonstrate that $λ_R$ is always computable by an algorithm which has access to coefficients of $R$, even when $J(R)$ is not computable. In the case when $R$ is a polynomial, Brolin-Lyubich measure coincides with the harmonic measure of the basin of infinity. We find a sufficient condition for computability of the harmonic measure of a domain, which holds for the basin of infinity of a polynomial mapping, and show that computability may fail for a general domain.

math.DS↗

Mating non-renormalizable quadratic polynomials

In this paper we prove the existence and uniqueness of matings of the basilica with any quadratic polynomial which lies outside of the 1/2-limb of M, is non-renormalizable, and does not have any non-repelling periodic orbits.

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Allometric metabolic scaling and fetal and placental weight

We tested the hypothesis that the fetal-placental relationship scales allometrically and identified modifying factors. Among women delivering after 34 weeks but prior to 43 weeks gestation, 24,601 participants in the Collaborative Perinatal Project (CPP) had complete data for placental gross proportion measures, specifically, disk shape, larger and smaller disk diameters and thickness, and umbilical cord length. The allometric metabolic equation was solved for alpha and beta by rewriting PW= alpha(BW)^beta as Log (PW) = Log(alpha) + beta*Log(BW). Mean beta was 0.78+ 0.02 (range 0.66, 0.89), 104% of that predicted by a supply-limited fractal system (0.75). Gestational age, maternal age, maternal BMI, parity, smoking, socioeconomic status, infant sex, and changes in placental proportions each had independent and significant effects on alpha. Conclusions: In the CPP cohort, the placental - birth weight relationship scales to approximately 3/4 power.

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Mean surface shape of a human placenta

The chorionic plate (or "fetal surface") of the human placenta is drawn as round, with the umbilical cord inserted roughly at the center, but variability of this shape is common. The average shape of the chorionic plate has never been established. The goal of this work is to measure the average shape in a birth cohort.

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Modeling the variability of shapes of a human placenta

While it is well-understood what a normal human placenta should look like, a deviation from the norm can take many possible shapes. In this paper we propose a mechanism for this variability based on the change in the structure of the vascular tree.

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Computability of Julia sets

In this paper we settle most of the open questions on algorithmic computability of Julia sets. In particular, we present an algorithm for constructing quadratics whose Julia sets are uncomputable. We also show that a filled Julia set of a polynomial is always computable.

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On computational complexity of Riemann mapping

In this paper we consider the computational complexity of uniformizing a domain with a given computable boundary. We give nontrivial upper and lower bounds in two settings: when the approximation of boundary is given either as a list of pixels, or by a Turing Machine.

math.CV↗

Cylinder renormalization of Siegel disks

We study one of the central open questions in one-dimensional renormalization theory -- the conjectural universality of golden-mean Siegel disks. We present an approach to the problem based on cylinder renormalization proposed by the second author. Numerical implementation of this approach relies on the Constructive Measurable Riemann Mapping Theorem proved by the first author. Our numerical study yields a convincing evidence to support the Hyperbolicity Conjecture in this setting.

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Constructing Non-Computable Julia Sets

We completely characterize the conformal radii of Siegel disks in the family $$P_θ(z)=e^{2πiθ}z+z^2,$$ corresponding to {\bf computable} parameters $θ$. As a consequence, we constructively produce quadratic polynomials with {\bf non-computable} Julia sets.

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Non-computable Julia sets

We show that under the definition of computability which is natural from the point of view of applications, there exist non-computable quadratic Julia sets.

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