arXiv subjects

Constanze Liaw

Publications and source records attributed to Constanze Liaw.

At least 37 records · Page 2Linked to original sources

arXiv2016

Moment Representations of Exceptional $X_1$ Orthogonal Polynomials

We obtain representations of $X_1$ exceptional orthogonal polynomials through determinants of matrices that have certain adjusted moments as entries. We start out directly from the Darboux transformation, allowing for a universal perspective, rather than dependent upon the particular system (Jacobi or Type of Laguerre polynomials). We include a recursion formula for the adjusted moments and provide the initial adjusted moments for each system. Throughout we relate to the various examples of $X_1$ exceptional orthogonal polynomials. We especially focus on and provide complete proofs for the Jacobi and the Type III Laguerre case, as they are less prevalent in literature. Lastly, we include a preliminary discussion explaining that the higher co-dimension setting becomes more involved. The number of possibilities and choices is exemplified, but only starts, with the lack of a canonical flag.

math.CA↗

arXiv2016

Zeros of optimal polynomial approximants: Jacobi matrices and Jentzsch-type theorems

We study the structure of the zeros of optimal polynomial approximants to reciprocals of functions in Hilbert spaces of analytic functions in the unit disk. In many instances, we find the minimum possible modulus of occurring zeros via a nonlinear extremal problem associated with norms of Jacobi matrices. We examine global properties of these zeros and prove Jentzsch-type theorems describing where they accumulate. As a consequence, we obtain detailed information regarding zeros of reproducing kernels in weighted spaces of analytic functions.

math.CA↗

arXiv2015

Orthogonal polynomials, reproducing kernels, and zeros of optimal approximants

We study connections between orthogonal polynomials, reproducing kernel functions, and polynomials $p$ minimizing Dirichlet-type norms $\|pf-1\|_α$ for a given function $f$. For $α\in [0,1]$ (which includes the Hardy and Dirichlet spaces of the disk) and general $f$, we show that such extremal polynomials are non-vanishing in the closed unit disk. For negative $α$, the weighted Bergman space case, the extremal polynomials are non-vanishing on a disk of strictly smaller radius, and zeros can move inside the unit disk. We also explain how $\mathrm{dist}_{D_α}(1,f\cdot \mathcal{P}_n)$, where $\mathcal{P}_n$ is the space of polynomials of degree at most $n$, can be expressed in terms of quantities associated with orthogonal polynomials and kernels, and we discuss methods for computing the quantities in question.

math.CV↗

arXiv2015

Moment Representations of the Exceptional $X_1$-Laguerre Orthogonal Polynomials

Exceptional orthogonal Laguerre polynomials can be viewed as an extension of the classical Laguerre polynomials per excluding polynomials of certain order(s) from being eigenfunctions for the corresponding exceptional differential operator. We are interested in the (so-called) Type I $X_1$-Laguerre polynomial sequence $\{L_n^α\}_{n=1}^\infty$, $\text{deg} \,p_n = n$ and $α>0$, where the constant polynomial is omitted. We derive two representations for the polynomials in terms of moments by using determinants. The first representation in terms of the canonical moments is rather cumbersome. We introduce adjusted moments and find a second, more elegant formula. We deduce a recursion formula for the moments and the adjusted ones. The adjusted moments are also expressed via a generating function. We observe a certain detachedness of the first two moments from the others.

math.CA↗

arXiv2015

Properties of vector-valued submodules on the bidisk

In previous work, the authors studied the compressed shift operators $S_{z_1}$ and $S_{z_2}$ on two-variable model spaces $H^2(\mathbb{D}^2)\ominus θH^2(\mathbb{D}^2)$, where $θ$ is a two-variable scalar inner function. Among other results, the authors used Agler decompositions to characterize the ranks of the operators $[S_{z_j}, S^*_{z_j}]$ in terms of the degree of rational $θ.$ In this paper, we examine similar questions for $H^2(\mathbb{D}^2)\ominus ΘH^2(\mathbb{D}^2)$ when $Θ$ is a matrix-valued inner function. We extend several results our previous work connecting $\text{Rank} [S_{z_j}, S^*_{z_j}]$ and the degree of $Θ$ to the matrix setting. When results do not clearly generalize, we conjecture what is true and provide supporting examples.

math.CV↗

arXiv2015

Singular integrals, rank one perturbations and Clark model in general situation

We start with considering rank one self-adjoint perturbations $A_α= A+α(\,\cdot\,,φ)φ$ with cyclic vector $φ\in \mathcal{H}$ on a separable Hilbert space $\mathcal H$. The spectral representation of the perturbed operator $A_α$ is realized by a (unitary) operator of a special type: the Hilbert transform in the two-weight setting, the weights being spectral measures of the operators $A$ and $A_α$. Similar results will be presented for unitary rank one perturbations of unitary operators, leading to singular integral operators on the circle. This motivates the study of abstract singular integral operators, in particular the regularization of such operator in very general settings. Further, starting with contractive rank one perturbations we present the Clark theory for arbitrary spectral measures (i.e. for arbitrary, possibly not inner characteristic functions). We present a description of the Clark operator and its adjoint in the general settings. Singular integral operators, in particular the so-called normalized Cauchy transform again plays a prominent role. Finally, we present a possible way to construct the Clark theory for dissipative rank one perturbations of self-adjoint operators. These lecture notes give an account of the mini-course delivered by the authors at the Thirteenth New Mexico Analysis Seminar and Afternoon in Honor of Cora Sadosky. Unpublished results are restricted to the last part of this manuscript.

math.FA↗

arXiv2015

Spectral analysis for the exceptional $X_m$-Jacobi equation

We provide the mathematical foundation for the $X_m$-Jacobi spectral theory. Namely, we present a self-adjoint operator associated to the differential expression with the exceptional $X_m$-Jacobi orthogonal polynomials as eigenfunctions. This proves that those polynomials are indeed eigenfunctions of the self-adjoint operator (rather than just formal eigenfunctions). Further, we prove the completeness of the exceptional $X_m$-Jacobi orthogonal polynomials (of degrees $m, m+1, m+2, ...$) in the Lebesgue--Hilbert space with the appropriate weight. In particular, the self-adjoint operator has no other spectrum.

math.CA↗

arXiv2014

Properties of Beurling-Type Submodules via Agler Decompositions

In this paper, we study operator-theoretic properties of the compressed shift operators $S_{z_1}$ and $S_{z_2}$ on complements of submodules of the Hardy space over the bidisk $H^2(\mathbb{D}^2)$. Specifically, we study Beurling-type submodules - namely submodules of the form $θH^2(\mathbb{D}^2)$ for $θ$ inner - using properties of Agler decompositions of $θ$ to deduce properties of $S_{z_1}$ and $S_{z_2}$ on model spaces $H^2(\mathbb{D}^2) \ominus θH^2(\mathbb{D}^2)$. Results include characterizations (in terms of $θ$) of when a commutator $[S_{z_j}^*, S_{z_j}]$ has rank $n$ and when subspaces associated to Agler decompositions are reducing for $S_{z_1}$ and $S_{z_2}$. We include several open questions.

math.CV↗

arXiv2014

Cyclic polynomials in two variables

We give a complete characterization of polynomials in two complex variables that are cyclic with respect to the coordinate shifts acting on Dirichlet-type spaces in the bidisk, which include the Hardy space and the Dirichlet space of the bidisk. The cyclicity of a polynomial depends on both the size and nature of the zero set of the polynomial on the distinguished boundary. The techniques in the proof come from real analytic function theory, determinantal representations for stable polynomials, and harmonic analysis on curves

math.FA↗

arXiv2014

A New Class of Exceptional Orthogonal Polynomials: The Type III $X_{m}$-Laguerre Polynomials And The Spectral Analysis of Three Types of Exceptional Laguerre Polynomials

The Bochner Classification Theorem (1929) characterizes the polynomial sequences $\p_{n}\}_{n=0}^{\infty}$, with $\text{deg}\,p_{n}=n$ that simultaneously form a complete set of eigenstates for a second-order differential operator and are orthogonal with respect to a positive Borel measure having finite moments of all orders. Indeed, up to a complex linear change of variable, only the classical Hermite, Laguerre, and Jacobi polynomials satisfy these conditions. In 2009, Gómez-Ullate, Kamran, and Milson found that for sequences $\{p_{n}\}_{n=1}^{\infty}$, $\text{deg}\,p_{n}=n$ (without the constant polynomial), the only such sequences are the exceptional $X_{1}$-Laguerre and $X_{1}$-Jacobi polynomials. Subsequently, other exceptional orthogonal polynomials $\{p_{n}\}_{n\in\mathbb{N}_{0}\diagdown A}$ were discovered and studied (here $A$ is a finite subset of the non-negative integers $\mathbb{N}_{0}$ and $\text{deg}\,p_{n}=n$ for all $n\in\mathbb{N}_{0}\diagdown A$). We call such a sequence an exceptional $X_{\left\vert A\right\vert}$ sequence. Remarkably, all exceptional sequences found, to date, form a complete orthogonal set in their natural Hilbert space setting. Among the exceptional sets already known are the Type I and Type II $X_{m}$-Laguerre polynomials, each omitting $m$ polynomials. We briefly discuss these polynomials and construct self-adjoint operators generated by their corresponding second-order differential expressions in appropriate Hilbert spaces. In addition, we present a new Type III family of $X_{m}$-Laguerre polynomials along with a detailed disquisition of its properties. We include several representations of these polynomials, orthogonality, norms, completeness, the location of their local extrema and roots, root asymptotics, as well as the spectral study of the second-order Type III exceptional $X_{m}$-Laguerre differential expression.

math.SP↗

arXiv2014

A Spectral Study of the Second-Order Exceptional $X_1$-Jacobi Differential Expression and a Related Non-classical Jacobi Differential Expression

The exceptional $X_{1}$-Jacobi differential expression is a second-order ordinary differential expression with rational coefficients; it was discovered by Gómez-Ullate, Kamran and Milson in 2009. In their work, they showed that there is a sequence of polynomial eigenfunctions $\left\{\widehat{P} _{n}^{(α,β)}\right\}_{n=1}^{\infty}$ called the exceptional $X_{1}$-Jacobi polynomials. There is no exceptional $X_{1}$-Jacobi polynomial of degree zero. These polynomials form a complete orthogonal set in the weighted Hilbert space $L^{2}((-1,1);\widehat{w}_{α,β}),$ where $\widehat{w}_{α,β}$ is a positive rational weight function related to the classical Jacobi weight. Among other conditions placed on the parameters $α$ and $β,$ it is required that $α,β>0.$ In this paper, we develop the spectral theory of this expression in $L^{2}((-1,1);\widehat{w}_{α,β})$. We also consider the spectral analysis of the `extreme' non-exceptional case, namely when $α=0$. In this case, the polynomial solutions are the non-classical Jacobi polynomials $\left\{ P_{n}^{(-2,β)}\right\} _{n=2}^{\infty}.$ We study the corresponding Jacobi differential expression in several Hilbert spaces, including their natural $L^{2}$ setting and a certain Sobolev space $S$ where the full sequence $\left\{ P_{n}^{(-2,β)}\right\} _{n=0}^{\infty}$ is studied and a careful spectral analysis of the Jacobi expression is carried out.

math.CA↗

arXiv2013

Delocalization for the 3-D discrete random Schroedinger operator at weak disorder

We apply a recently developed approach (Liaw 2013) to study the existence of extended states for the three dimensional discrete random Schroedinger operator at small disorder. The conclusion of delocalization at small disorder agrees with other numerical and experimental observations. Further the work furnishes a verification of the numerical approach and its implementation. Not being based on scaling theory, this method eliminates problems due to boundary conditions, common to previous numerical methods in the field. At the same time, as with any numerical experiment, one cannot exclude finite-size effects with complete certainty. Our work can be thought of as a new and quite different use of Lanczos' algorithm; a posteriori tests to show that the orthogonality loss is very small. We numerically track the "bulk distribution" (here: the distribution of where we most likely find an electron) of a wave packet initially located at the origin, after iterative application of the discrete random Schroedinger operator.

math-ph↗

arXiv2013

Cyclicity in Dirichlet-type spaces and extremal polynomials II: functions on the bidisk

We study Dirichlet-type spaces $\mathfrak{D}_α$ of analytic functions in the unit bidisk and their cyclic elements. These are the functions $f$ for which there exists a sequence $(p_n)_{n=1}^{\infty}$ of polynomials in two variables such that $\|p_nf-1\|_α\to 0$ as $n\to \infty$. We obtain a number of conditions that imply cyclicity, and obtain sharp estimates on the best possible rate of decay of the norms $\|p_nf-1\|_α$, in terms of the degree of $p_n$, for certain classes of functions using results concerning Hilbert spaces of functions of one complex variable and comparisons between norms in one and two variables. We give examples of polynomials with no zeros on the bidisk that are not cyclic in $\mathfrak{D}_α$ for $α>1/2$ (including the Dirichlet space); this is in contrast with the one-variable case where all non-vanishing polynomials are cyclic in Dirichlet-type spaces that are not algebras ($α\le 1$). Further, we point out the necessity of a capacity zero condition on zero sets (in an appropriate sense) for cyclicity in the setting of the bidisk, and conclude by stating some open problems.

math.FA↗

arXiv2013

Clark model in general situation

For a unitary operator the family of its unitary perturbations by rank one operators with fixed range is parametrized by a complex parameter $γ, |γ|=1$. Namely all such unitary perturbations are $U_γ:=U+(γ-1) (., b_1)_{\mathcal H} b$, where $b\in\mathcal H, \|b\|=1, b_1=U^{-1} b, |γ|=1$. For $|γ|<1$ operators $U_γ$ are contractions with one-dimensional defects. Restricting our attention on the non-trivial part of perturbation we assume that $b$ is cyclic for $U$. Then the operator $U_γ$, $|γ|<1$ is a completely non-unitary contraction, and thus unitarily equivalent to its functional model $\mathcal M_γ$, which is the compression of the multiplication by the independent variable $z$ onto the model space $\mathcal K_{θ_γ}$, where $θ_γ$ is the characteristic function of the contraction $U_γ$. The Clark operator $Φ_γ$ is a unitary operator intertwining $U_γ, |γ|<1$ and its model $\mathcal M_γ$, $\mathcal M_γΦ_γ= Φ_γU_γ$. If spectral measure of $U$ is purely singular (equivalently, $θ_γ$ is inner), operator $Φ_γ$ was described from a slightly different point of view by D. Clark. When $θ_γ$ is extreme point of the unit ball in $H^\infty$ was treated by D. Sarason using the sub-Hardy spaces introduced by L. de Branges. We treat the general case and give a systematic presentation of the subject. We find a formula for the adjoint operator $Φ^*_γ$ which is represented by a singular integral operator, generalizing the normalized Cauchy transform studied by A. Poltoratskii. We present a "universal" representation that works for any transcription of the functional model. We then give the formulas adapted for the Sz.-Nagy--Foias and de Branges--Rovnyak transcriptions, and finally obtain the representation of $Φ_γ$.

math.FA↗

arXiv2013

Cyclicity in rank-one perturbation problems

The property of cyclicity of a linear operator, or equivalently the property of simplicity of its spectrum, is an important spectral characteristic that appears in many problems of functional analysis and applications to mathematical physics. In this paper we study cyclicity in the context of rank-one perturbation problems for self-adjoint and unitary operators. We show that for a fixed non-zero vector the property of being a cyclic vector is not rare, in the sense that for any family of rank-one perturbations of self-adjoint or unitary operators acting on the space, that vector will be cyclic for every operator from the family, with a possible exception of a small set with respect to the parameter. We discuss applications of our results to Anderson-type Hamiltonians.

math-ph↗

arXiv2013

Cyclicity in Dirichlet-type spaces and extremal polynomials

For functions $f$ in Dirichlet-type spaces we study how to determine constructively optimal polynomials $p_n$ that minimize $\|p f-1\|_α$ among all polynomials $p$ of degree at most $n$. Then we give upper and lower bounds for the rate of decay of $\|p_{n}f-1\|_α$ as $n$ approaches $\infty$. Further, we study a generalization of a weak version of the Brown-Shields conjecture and some computational phenomena about the zeros of optimal polynomials.

math.CA↗

arXiv2012

Conductance for the two dimensional discrete random Schroedinger operator with small disorder

As part of condensed-matter physics, the field of Anderson localization concerns the study of conductance of electrons in a random medium. We summarize and explain the results obtained in "A new numerical approach to Anderson (de)localization" from a physics perspective. Further, we discover a relationship with the so-called Thouless criterion - a dimensionless scaling parameter often used as the sole indicator whether or not the system exhibits Anderson localization.

math-ph↗

arXiv2012

A new numerical approach to Anderson (de)localization

We develop a new approach for the Anderson localization problem. The implementation of this method yields strong numerical evidence leading to a (surprising to many) conjecture: The two dimensional discrete random Schroedinger operator with small disorder allows states that are dynamically delocalized with positive probability. This approach is based on a recent result by Abakumov-Liaw-Poltoratski which is rooted in the study of spectral behavior under rank-one perturbations, and states that every non-zero vector is almost surely cyclic for the singular part of the operator. The numerical work presented is rather simplistic compared to other numerical approaches in the field. Further, this method eliminates effects due to boundary conditions. While we carried out the numerical experiment almost exclusively in the case of the two dimensional discrete random Schroedinger operator, we include the setup for the general class of Anderson models called Anderson-type Hamiltonians. We track the location of the energy when a wave packet initially located at the origin is evolved according to the discrete random Schroedinger operator. This method does not provide new insight on the energy regimes for which diffusion occurs.

math-ph↗