Non-negative hereditary polynomials in a free *−algebra

We prove a non-negative-stellensatz and a null-stellensatz for a class of polynomials called hereditary polynomials in a free *-algebra.Partially supported by NSF, DARPA and Ford Motor Co.Partially supported by NSF grant DMS-0140112Partially supported by NSF grant DMS-0100367     

Szeg? type polynomials and para-orthogonal polynomials

Szeg? type polynomials with respect to a linear functional M for which the moments M[tn]=μ−n are all complex, μ−n=μn and Dn≠0 for n?0, are considered. Here, Dn are the associated Toeplitz determinants. Para-orthogonal polynomials are also studied without relying on any integral representation. Relation between the Toeplitz determinants of two different types of moment functionals are given. Starting from the existence of polynomials similar to para-orthogonal polynomials, sufficient conditions for the existence of Szeg? type polynomials are also given. Examples are provided to justify the results.     

Discrete orthogonal polynomials – polynomial modification of a classical functional

Polynomial modifications of a classical discrete linear functional are examined in detail, in particular when the new linear functional remains classical. New addition formulas are deduced for Charlier, Meixner and Hahn polynomials from the Christoffei representation and results are also given for a particular generalized Meixner family.     

Local connectivity of Julia sets for a family of biquadratic polynomials

The local connectivity of Julia sets for the family of biquadratic polynomials f_c(z)= (z~2-2c~2)z~2 with a parameter c is discussed.It is proved that for any parameter c,the boundary of the immediately attracting domain of f_c is a Jordan curve.     

Nikol’skii inequality for algebraic polynomials on a multidimensional Euclidean sphere

We study the sharp Nikol’skii inequality between the uniform norm and the L _q norm of algebraic polynomials of a given (total) degree n ≥ 1 on the unit sphere \(\mathbb{S}^{m - 1} \) of the Euclidean space ? ^m for 1 ≤ q < ∞. We prove that the polynomial ? _n in one variable with unit leading coefficient that deviates least from zero in the space L _q ^ψ (?1, 1) of functions f such that |f| ^q is summable over (?1, 1) with the Jacobi weight ψ(t) = (1 - t)^α(1 + t)^β, α = (m - 1)/2, β = (m - 3)/2 as a zonal polynomial in one variable t = ξ _m , where x = (ξ ₁, ξ ₂, …, ξ _m ) ∈ \(\mathbb{S}^{m - 1} \), is (in a certain sense, unique) extremal polynomial in the Nikol’skii inequality on the sphere \(\mathbb{S}^{m - 1} \). The corresponding one-dimensional inequalities for algebraic polynomials on a closed interval are discussed.     

Global asymptotics for Laguerre polynomials with large negative parameter—a Riemann-Hilbert approach

In this paper, we study the asymptotic behavior of the Laguerre polynomials as n→∞. Here α _n is a sequence of negative numbers and −α _n /n tends to a limit A>1 as n→∞. An asymptotic expansion is obtained, which is uniformly valid in the upper half plane ℂ₊={z:Im z≥0}. A corresponding expansion is also given for the lower half plane ℂ₋={z:Im z≤0}. The two expansions hold, in particular, in regions containing the curve Γ in the complex plane, on which these polynomials are orthogonal. Our method is based on the Riemann-Hilbert approach introduced by Deift and Zhou. The work of R. Wong is partially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. CityU 102504).     

Global asymptotic expansions of the Laguerre polynomials—a Riemann–Hilbert approach

By using the steepest descent method for Riemann–Hilbert problems introduced by Deift–Zhou (Ann Math 137:295–370, 1993), we derive two asymptotic expansions for the scaled Laguerre polynomial $L^{(\alpha)}_n(\nu z)$ as n→∞, where ν=4n+2α+2. One expansion holds uniformly in a right half-plane $\text{Re}\; z\geq \delta_1, 0<\delta_1<1$ , which contains the critical point z=1; the other expansion holds uniformly in a left half-plane $\text{Re}\; z\leq 1-\delta_2, 0<\delta_2<1-\delta_1$ , which contains the other critical point z=0. The two half-planes together cover the entire complex z-plane. The critical points z=1 and z=0 correspond, respectively, to the turning point and the singularity of the differential equation satisfied by $L^{(\alpha)}_n(\nu z)$ .     

Schröder iteration functions associated with a one-parameter family of biquadratic polynomials

Schröder iteration functions, a generalization of the Newton–Raphson method to determine roots of equations, are generally rational functions which possess some critical points, free to converge to attracting cycles. These free critical points, however, satisfy some higher-degree polynomial equations which we solve analytically. Then, with the help of microcomputer plots, we examine the Julia sets of the Schröder functions and the orbits of all their free critical points associated with a particular one-parameter family of quartic polynomials, by walking in their dynamic and parameter spaces. This examination takes place in the complex plane as well as on the Riemann sphere.     

Approximation of analytic functions by polynomials “close” to polynomials of best approximation

The rate of approximation of analytic functions at interior points of compact sets with connected complement by polynomials close to polynomials of best approximation is investigated.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 44, No. 2, pp. 208–214, February, 1992.     

Heat “polynomials” for a singular differential operator on (0, ∞)

We generalize the theory of the heat polynomials introduced by P. V. Rosenbloom and D. V. Widder for a more general class of singular differential operator on (0, ). The heat polynomials associated with the Bessel operator and studied by D. T. Haimo appear as a particular case in this paper. In the special cases of second derivative and Bessel operators the heat polynomials are in fact polynomials inx andt, however, this property does not hold in general.Communicated by Tom. H. Koornwinder.     

Elliptic polynomials orthogonal on the unit circle with a dense point spectrum

We introduce two explicit examples of polynomials orthogonal on the unit circle. Moments and the reflection coefficients are expressed in terms of the Jacobi elliptic functions. We find explicit expression for these polynomials in terms of elliptic hypergeometric functions. We show that the obtained polynomials are orthogonal on the unit circle with respect to a dense point measure. We also construct corresponding explicit systems of polynomials orthogonal on the interval of the real axis with respect to a dense point measure. They can be considered as an elliptic generalization of the Askey-Wilson polynomials of a special type.      

When do linear combinations of orthogonal polynomials yield new sequences of orthogonal polynomials?

Given {P_n}_n≥0 a sequence of monic orthogonal polynomials, we analyze their linear combinations with constant coefficients and fixed length, i.e.,