Relative asymptotic of multiple orthogonal polynomials for Nikishin systems

We prove the relative asymptotic behavior for the ratio of two sequences of multiple orthogonal polynomials with respect to the Nikishin systems of measures. The first Nikishin system is such that for each k, σ_k has a constant sign on its compact support consisting of an interval , on which almost everywhere, and a discrete set without accumulation points in . If denotes the smallest interval containing , we assume that Δ_k∩Δ_k+1=0/, k=1,…,m−1. The second Nikishin system is a perturbation of the first by means of rational functions r_k, k=1,…,m, whose zeros and poles lie in .     

A canonical family of multiple orthogonal polynomials for Nikishin systems

For any pair of compact intervals of the real line Δ₁, Δ₂, with Δ₁∩Δ₂=∅, we obtain two probability measures μ₁, τ₁, supported on Δ₁ and Δ₂ respectively, such that the Nikishin system N(μ₁,τ₁) has a sequence of monic multiple orthogonal polynomials which satisfy a four term recurrence relation with constant coefficients of period 2. The measures are obtained from the functions which give the ratio asymptotic of multiple orthogonal polynomials with respect to an arbitrary Nikishin system N(σ₁,σ₂) on Δ₁, Δ₂, such that a.e. on Δi, i=1,2. The role of μ₁, τ₁ is symmetric in the sense that the same construction is possible on Δ₂, Δ₁, with N(τ₁,μ₁).     

Dynamics and interpretation of some integrable systems via multiple orthogonal polynomials

High-order non-symmetric difference operators with complex coefficients are considered. The correspondence between dynamics of the coefficients of the operator defined by a Lax pair and its resolvent function is established. The method of investigation is based on the analysis of the moments for the operator. The solution of a discrete dynamical system is studied. We give explicit expressions for the resolvent function and, under some conditions, the representation of the vector of functionals, associated with the solution for the integrable systems.     

Differential polynomials and normal families

This paper continues the researches of Yang Lo and others, and gives a further generalization of Gu's criterion for the normality of a family of meromorphic functions.     

Normal forms and linearization of resonant vector fields with multiple eigenvalues

We discuss the linearization and normal forms of resonant vector fields X(x)=Ax+a(x), where A has one double or triple eigenvalue or a pair of double eigenvalues: we present a simple way of identifying the resonant monomials that have to appear in its normal form, and also effective conditions on the nonlinearity a(x) for the resonant vector field to be linearizable.     

Adomian decomposition method with orthogonal polynomials: Legendre polynomials

This paper illustrates the using of orthogonal polynomials to modify the Adomian decomposition method. The method of employing Legendre polynomials to improve the Adomian decomposition method is presented here and compared to the method of using Chebyshev polynomials. The presented modified Adomian decomposition method is validated through an example and advantage as well as efficiency of this method is verified through investigating and comparing the results. In this paper, it is concluded that both orthogonal polynomials: Chebyshev and Legendre polynomials can be successfully used for the Adomian decomposition method and comparatively the Chebyshev expansion provides the better estimation.     

Moment systems and orthogonal polynomials in several variables

The first part of this paper deals with general moment (“Appell”) systems on $\text{R}$^N generated by a Hamiltonian function H(x, D) and also with representations of GL(N) on the associated spaces of polynomials. The second part discusses the theory of Bernoulli generators on $\text{R}$^N determining systems of orthogonal polynomials that are extensions of the Meixner polynomials to several variables. Linear actions for these spaces are discussed. Some tensors related to the general Bernoulli generators are considered.     

Differential polynomials with dilations in the argument and normal families

We show that a family ${\mathcal{F}}$ of analytic functions in the unit disk ${\mathbb{D}}$ which satisfy a condition of the form $$ f^n(z)+P[f](xz)+b\ne 0 $$ for all ${f\in\mathcal{F}}$ and all ${z\in\mathbb{D}}$ (where n ?? 3, 0?<?|x| ?? 1, b ?? 0 and P is an arbitrary differential polynomial of degree at most n ? 2 with constant coefficients and without terms of degree 0) is normal at the origin. Under certain additional assumptions on P the same holds also for b?=?0. The proof relies on a modification of Nevanlinna theory in combination with the Zalcman?CPang rescaling method. Furthermore we prove some corresponding results of Picard type for functions meromorphic in the plane.     

The asymptotic zero distribution of multiple orthogonal polynomials associated with Macdonald functions

We study the asymptotic zero distribution of type II multiple orthogonal polynomials associated with two Macdonald functions (modified Bessel functions of the second kind). On the basis of the four-term recurrence relation, it is shown that, after proper scaling, the sequence of normalized zero counting measures converges weakly to the first component of a vector of two measures which satisfies a vector equilibrium problem with two external fields. We also give the explicit formula for the equilibrium vector in terms of solutions of an algebraic equation.     

Generalized normal forms for two-dimensional systems of ordinary differential equations with linear and quadratic unperturbed parts

Various definitions of normal forms for systems of ordinary differential equations are discussed. The notion of a generalized normal form and the problem of formal equivalency of systems of differential equations in terms of resonant equations are considered. The method of resonant equations is applied to two-dimensional systems whose unperturbed parts are linear in the first equation and quadratic in the second one.     

Absolutely continuous measures for systems of orthogonal polynomials with unbounded recurrence coefficients

This paper considers systems of orthogonal polynomials which satisfy a three-term recurrence relation in which the recursion coefficients are unbounded. Conditions are imposed on the coefficient sequences to establish that the corresponding measure of orthogonality is absolutely continuous.     

Cumulants, lattice paths, and orthogonal polynomials

A formula expressing free cumulants in terms of Jacobi parameters of the corresponding orthogonal polynomials is derived. It combines Flajolet's theory of continued fractions and the Lagrange inversion formula. For the converse we discuss Gessel–Viennot theory to express Hankel determinants in terms of various cumulants.     

On superquadratic periodic systems with indefinite linear part

We consider a second order periodic system with an indefinite linear part and a potential function which is superquadratic but does not satisfy the AR-condition. Using Morse critical groups, we show that the system has at least one nontrivial solution.