We use cookies to distinguish you from other users and to provide you with a better experience on our websites. Close this message to accept cookies or find out how to manage your cookie settings.
To save content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about saving content to .
To save content items to your Kindle, first ensure coreplatform@cambridge.org
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
For any real polynomial
$p(x)$
of even degree k, Shapiro [‘Problems around polynomials: the good, the bad and the ugly
$\ldots $
’, Arnold Math. J.1(1) (2015), 91–99] proposed the conjecture that the sum of the number of real zeros of the two polynomials
$(k-1)(p{'}(x))^{2}-kp(x)p{"}(x)$
and
$p(x)$
is larger than 0. We prove that the conjecture is true except in one case: when the polynomial
$p(x)$
has no real zeros, the derivative polynomial
$p{'}(x)$
has one real simple zero, that is,
$p{'}(x)=C(x)(x-w)$
, where
$C(x)$
is a polynomial with
$C(w)\ne 0$
, and the polynomial
$(k-1)(C(x))^2(x-w)^{2}-kp(x)C{'}(x)(x-w)-kC(x)p(x)$
has no real zeros.
This paper mainly considers the problem of generalizing a certain class of analytic functions by means of a class of difference operators. We consider some relations between starlike or convex functions and functions belonging to such classes. Some other useful properties of these classes are also considered.
where $h$ is a convex univalent function with $0\in h(\mathbb {D}).$ The proof of the main result is based on the original lemma for convex univalent functions and offers a new approach in the theory. In particular, the above differential subordination leads to generalizations of the well-known Briot-Bouquet differential subordination. Appropriate applications among others related to the differential subordination of harmonic mean are demonstrated. Related problems concerning differential equations are indicated.
Let
$ \mathcal {B} $
be the class of analytic functions
$ f $
in the unit disk
$ \mathbb {D}=\{z\in \mathbb {C} : |z|<1\} $
such that
$ |f(z)|<1 $
for all
$ z\in \mathbb {D} $
. If
$ f\in \mathcal {B} $
of the form
$ f(z)=\sum _{n=0}^{\infty }a_nz^n $
, then
$ \sum _{n=0}^{\infty }|a_nz^n|\leq 1 $
for
$ |z|=r\leq 1/3 $
and
$ 1/3 $
cannot be improved. This inequality is called Bohr inequality and the quantity
$ 1/3 $
is called Bohr radius. If
$ f\in \mathcal {B} $
of the form
$ f(z)=\sum _{n=0}^{\infty }a_nz^n $
, then
$ |\sum _{n=0}^{N}a_nz^n|<1\;\; \mbox {for}\;\; |z|<{1}/{2} $
and the radius
$ 1/2 $
is the best possible for the class
$ \mathcal {B} $
. This inequality is called Bohr–Rogosinski inequality and the corresponding radius is called Bohr–Rogosinski radius. Let
$ \mathcal {H} $
be the class of all complex-valued harmonic functions
$ f=h+\bar {g} $
defined on the unit disk
$ \mathbb {D} $
, where
$ h $
and
$ g $
are analytic in
$ \mathbb {D} $
with the normalization
$ h(0)=h^{\prime }(0)-1=0 $
and
$ g(0)=0 $
. Let
$ \mathcal {H}_0=\{f=h+\bar {g}\in \mathcal {H} : g^{\prime }(0)=0\}. $
For
$ \alpha \geq 0 $
and
$ 0\leq \beta <1 $
, let
be a class of close-to-convex harmonic mappings in
$ \mathbb {D} $
. In this paper, we prove the sharp Bohr–Rogosinski radius for the class
$ \mathcal {W}^{0}_{\mathcal {H}}(\alpha , \beta ) $
.
Granville recently asked how the Mahler measure behaves in the context of polynomial dynamics. For a polynomial
$f(z)=z^d+\cdots \in {\mathbb C}[z],\ \deg (f)\ge 2,$
we show that the Mahler measure of the iterates
$f^n$
grows geometrically fast with the degree
$d^n,$
and find the exact base of that exponential growth. This base is expressed via an integral of
$\log ^+|z|$
with respect to the invariant measure of the Julia set for the polynomial
$f.$
Moreover, we give sharp estimates for such an integral when the Julia set is connected.
The purpose of this paper is to initiate a theory concerning the dynamics of asymptotically holomorphic polynomial-like maps. Our maps arise naturally as deep renormalizations of asymptotically holomorphic extensions of $C^r$ ($r>3$) unimodal maps that are infinitely renormalizable of bounded type. Here we prove a version of the Fatou–Julia–Sullivan theorem and a topological straightening theorem in this setting. In particular, these maps do not have wandering domains and their Julia sets are locally connected.
We prove several sharp distortion and monotonicity theorems for spherically convex functions defined on the unit disk involving geometric quantities such as spherical length, spherical area, and total spherical curvature. These results can be viewed as geometric variants of the classical Schwarz lemma for spherically convex functions.
We consider an analogue of Kontsevich’s matrix Airy function where the cubic potential
$\textrm{Tr}(\Phi^3)$
is replaced by a quartic term
$\textrm{Tr}\!\left(\Phi^4\right)$
. Cumulants of the resulting measure are known to decompose into cycle types for which a recursive system of equations can be established. We develop a new, purely algebraic geometrical solution strategy for the two initial equations of the recursion, based on properties of Cauchy matrices. These structures led in subsequent work to the discovery that the quartic analogue of the Kontsevich model obeys blobbed topological recursion.
We characterize zero sets for which every subset remains a zero set too in the Fock space $\mathcal {F}^p$, $1\leq p<\infty $. We are also interested in the study of a stability problem for some examples of uniqueness set with zero excess in Fock spaces.
Two boundary value problems are solved for potential steady-state 2D Darcian seepage flows towards a line sink in a homogeneous isotropic soil from a ponded land surface, which is not flat but profiled. The aim of this shaping is ‘uniformisation’ of the velocity and travel time between this surface and a horizontal drain modelled by a line sink. The complex potential domain is a half-strip, which is mapped onto a reference plane. Either the velocity magnitude or a vertical coordinate along the land surface are control variables. Either a complexified velocity or complex physical coordinate is reconstructed by solving mixed boundary-value problems with the help of the Keldysh-Sedov formula via singular integrals, the kernel of which are the control functions. The flow nets, isotachs and breakthrough curves are found by computer algebra routines. A designed soil hump above the drain ameliorates an unwanted ‘preferential flow’ (shortcut) and improves leaching of salinised soil of a cropfield during a pre-cultivation season.
For a domain G in the one-point compactification
$\overline{\mathbb{R}}^n = {\mathbb{R}}^n \cup \{ \infty\}$
of
${\mathbb{R}}^n, n \geqslant 2$
, we characterise the completeness of the modulus metric
$\mu_G$
in terms of a potential-theoretic thickness condition of
$\partial G\,,$
Martio’s M-condition [35]. Next, we prove that
$\partial G$
is uniformly perfect if and only if
$\mu_G$
admits a minorant in terms of a Möbius invariant metric. Several applications to quasiconformal maps are given.
In this paper, we consider the family of nth degree polynomials whose coefficients form a log-convex sequence (up to binomial weights), and investigate their roots. We study, among others, the structure of the set of roots of such polynomials, showing that it is a closed convex cone in the upper half-plane, which covers its interior when n tends to infinity, and giving its precise description for every
$n\in \mathbb {N}$
,
$n\geq 2$
. Dual Steiner polynomials of star bodies are a particular case of them, and so we derive, as a consequence, further properties for their roots.
We extend our study of variability regions, Ali et al. [‘An application of Schur algorithm to variability regions of certain analytic functions–I’, Comput. Methods Funct. Theory, to appear] from convex domains to starlike domains. Let
$\mathcal {CV}(\Omega )$
be the class of analytic functions f in
${\mathbb D}$
with
$f(0)=f'(0)-1=0$
satisfying
$1+zf''(z)/f'(z) \in {\Omega }$
. As an application of the main result, we determine the variability region of
$\log f'(z_0)$
when f ranges over
$\mathcal {CV}(\Omega )$
. By choosing a particular
$\Omega $
, we obtain the precise variability regions of
$\log f'(z_0)$
for some well-known subclasses of analytic and univalent functions.
The Julia set of the exponential family
$E_{\kappa }:z\mapsto \kappa e^z$
,
$\kappa>0$
was shown to be the entire complex plane when
$\kappa>1/e$
essentially by Misiurewicz. Later, Devaney and Krych showed that for
$0<\kappa \leq 1/e$
the Julia set is an uncountable union of pairwise disjoint simple curves tending to infinity. Bergweiler generalized the result of Devaney and Krych for a three-dimensional analogue of the exponential map called the Zorich map. We show that the Julia set of certain Zorich maps with symmetry is the whole of
$\mathbb {R}^3$
, generalizing Misiurewicz’s result. Moreover, we show that the periodic points of the Zorich map are dense in
$\mathbb {R}^3$
and that its escaping set is connected, generalizing a result of Rempe. We also generalize a theorem of Ghys, Sullivan and Goldberg on the measurable dynamics of the exponential.
We begin the study of Hankel matrices whose entries are logarithmic coefficients of univalent functions and give sharp bounds for the second Hankel determinant of logarithmic coefficients of convex and starlike functions.
The classical model for studying one-phase Hele-Shaw flows is based on a highly nonlinear moving boundary problem with the fluid velocity related to pressure gradients via a Darcy-type law. In a standard configuration with the Hele-Shaw cell made up of two flat stationary plates, the pressure is harmonic. Therefore, conformal mapping techniques and boundary integral methods can be readily applied to study the key interfacial dynamics, including the Saffman–Taylor instability and viscous fingering patterns. As well as providing a brief review of these key issues, we present a flexible numerical scheme for studying both the standard and nonstandard Hele-Shaw flows. Our method consists of using a modified finite-difference stencil in conjunction with the level-set method to solve the governing equation for pressure on complicated domains and track the location of the moving boundary. Simulations show that our method is capable of reproducing the distinctive morphological features of the Saffman–Taylor instability on a uniform computational grid. By making straightforward adjustments, we show how our scheme can easily be adapted to solve for a wide variety of nonstandard configurations, including cases where the gap between the plates is linearly tapered, the plates are separated in time, and the entire Hele-Shaw cell is rotated at a given angular velocity.
We consider the problem of computing the partition function
$\sum _x e^{f(x)}$
, where
$f: \{-1, 1\}^n \longrightarrow {\mathbb R}$
is a quadratic or cubic polynomial on the Boolean cube
$\{-1, 1\}^n$
. In the case of a quadratic polynomial f, we show that the partition function can be approximated within relative error
$0 < \epsilon < 1$
in quasi-polynomial
$n^{O(\ln n - \ln \epsilon )}$
time if the Lipschitz constant of the non-linear part of f with respect to the
$\ell ^1$
metric on the Boolean cube does not exceed
$1-\delta $
, for any
$\delta>0$
, fixed in advance. For a cubic polynomial f, we get the same result under a somewhat stronger condition. We apply the method of polynomial interpolation, for which we prove that
$\sum _x e^{\tilde {f}(x)} \ne 0$
for complex-valued polynomials
$\tilde {f}$
in a neighborhood of a real-valued f satisfying the above mentioned conditions. The bounds are asymptotically optimal. Results on the zero-free region are interpreted as the absence of a phase transition in the Lee–Yang sense in the corresponding Ising model. The novel feature of the bounds is that they control the total interaction of each vertex but not every single interaction of sets of vertices.
Let f and g be two quasiregular maps in $\mathbb{R}^d$ that are of transcendental type and also satisfy
$f\circ g =g \circ f$
. We show that if the fast escaping sets of those functions are contained in their respective Julia sets then those two functions must have the same Julia set. We also obtain the same conclusion about commuting quasimeromorphic functions with infinite backward orbit of infinity. Furthermore we show that permutable quasiregular functions of the form f and
$g = \phi \circ f$
, where
$\phi$
is a quasiconformal map, have the same Julia sets and that polynomial type quasiregular maps cannot commute with transcendental type ones unless their degree is less than or equal to their dilatation.
En s’appuyant sur la notion d’équivalence au sens de Bohr entre polynômes de Dirichlet et sur le fait que sur un corps quadratique la fonction zeta de Dedekind peut s’écrire comme produit de la fonction zeta de Riemann et d’une fonction L, nous montrons que, pour certaines valeurs du discriminant du corps quadratique, les sommes partielles de la fonction zeta de Dedekind ont leurs zéros dans des bandes verticales du plan complexe appelées bandes critiques et que les parties réelles de leurs zéros y sont denses.
The Neumann–Poincaré (NP) operator, a singular integral operator on the boundary of a domain, naturally appears when one solves a conductivity transmission problem via the boundary integral formulation. Recently, a series expression of the NP operator was developed in two dimensions based on geometric function theory [34]. In this paper, we investigate geometric properties of composite materials using this series expansion. In particular, we obtain explicit formulas for the polarisation tensor and the effective conductivity for an inclusion or a periodic array of inclusions of arbitrary shape with extremal conductivity, in terms of the associated exterior conformal mapping. Also, we observe by numerical computations that the spectrum of the NP operator has a monotonic behaviour with respect to the shape deformation of the inclusion. Additionally, we derive inequality relations of the coefficients of the Riemann mapping of an arbitrary Lipschitz domain using the properties of the polarisation tensor corresponding to the domain.