Quantum zonal spherical functions and Macdonald polynomials

A unified theory of quantum symmetric pairs is applied to q-special functions. Previous work characterized certain left coideal subalgebras in the quantized enveloping algebra and established an appropriate framework for quantum zonal spherical functions. Here a distinguished family of such functions, invariant under the Weyl group associated to the restricted roots, is shown to be a family of Macdonald polynomials, as conjectured by Koornwinder and Macdonald. Our results place earlier work for Lie algebras of classical type in a general context and extend to the exceptional cases.     

Runge-Kutta stability on a Floquet problem

This work examines the stability of explicit Runge-Kutta methods applied to a certain linear ordinary differential equation with periodic coefficients. On this problem naïve use of the eigenvalues of the Jacobian results in misleading conclusions about stable behaviour. It is shown, however, that a valid analogue of the classical absolute stability theory can be developed. Further, using a suitable generalisation of the equilibrium theory of Hall [ACM Trans. on Math. Soft. 11 (1985), pp. 289–301], accurate predictions are made about the performance of modern, adaptive algorithms.Supported by the University of Dundee Research Initiatives Fund.     

Floquet theory for left-definite Sturm-Liouville problems

We determine the essential spectrum of left-definite Sturm-Liouville problems with periodic coefficients and a weight function which changes sign.     

Floquet operators without singular continuous spectrum

Let U be a unitary operator defined on a infinite-dimensional separable complex Hilbert space H. Assume there exists a self-adjoint operator A on H such that

U^∗AU−A?cI+K     

A note on analyticity and Floquet isospectrality

A simple argument shows that certain complex Hill's operators have the same Floquet multipliers as the zero potential case. Previous results are extended to include matrix coefficients and some meromorphic potentials.

     

拟周期系统的Floquet理论

在这篇文章,我们对拟周期系统dx/dt=A(ω₁t,ω₂t.…,ω_mt)x (0.1)建立了Floquet理论.其中n×n方阵A(u₁,u₂,…,u_m)是u₁,u₂,…,u_m以2π为周期的周期方阵,同时假定A(u₁,u₂,…,u_m)∈Cτ,τ=(N+1)τ₀,τ₀=2(m+1),N=1/2n(n+1).我们定义了(0.1)的特征指数根β₁,β₂,…,β_n,假设下式成立:其中K(ω),K(ω,β)>0,k_μ,i_v是整数,k₁,k₂…,k_m不全为零:i2=-1.那末有拟周期线性变换,把(0.1)化为常系数的线性系统.     

Floquet theory for a Volterra integro-dynamic system

In this paper, we study periodic linear Volterra systems on periodic time scales which include not only discrete and continuous dynamical systems but also systems with a mixture of discrete and continuous parts (e.g. hybrid dynamical systems). We discuss the relationship between the solution of the Volterra integro-dynamic system and the limiting equation of the corresponding system. We also develop integrability conditions of the resolvent of Volterra integro-dynamic systems.     

Floquet theorem for linear implicit nonautonomous difference systems

The aim of this paper is to develop the Floquet theory for linear implicit difference systems (LIDS). It is proved that any index-1 LIDS can be transformed into its Kronecker normal form. Then the Floquet theorem on the representation of the fundamental matrix of index-1 periodic LIDS has been established. As an immediate consequence, the Lyapunov reduction theorem is proved. Some applications of the obtained results are discussed.     

m-functions and Floquet exponents for linear differential systems

Sunto Si definisce un esponente di Floquet per certe equazioni differenziali lineari nonperiodiche, la parte immaginaria del quale rappresenta una «rotazione» delle soluzioni di dette equazioni. Inoltre si discute la relazione fra l'esponente di Floquet e le funzioni m di Weyl-Kodaira, e fra la rotazione e certi problemi spettrali.

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The author would like to thank Dr. RichardCushman for stimulating conversations on the subjects considered in this paper.     

Singular continuous Floquet operator for periodic quantum systems

Consider the Floquet operator of a time independent quantum system, acting on a separable Hilbert space, periodically perturbed by a rank one kick: e−iH₀Te−iκT|?〉〈?| where T is the period, κ the coupling constant, and H₀ is a pure point self-adjoint operator, bounded from below. Under some hypotheses on the vector ?, cyclic w.r.t. H₀ we prove the following:

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If the gaps between the eigenvalues (λn) are such that λn+1−λn?Cn−γ for some γ∈]0,1[ and C>0, then the Floquet operator of the perturbed system is purely singular continuous T-a.e.

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If H₀ is the Hamiltonian of the one-dimensional rotator on L²(R/T₀Z) and the ratio is irrational, then the Floquet operator is purely singular continuous as soon as κT≠0(2π).

We also establish an integral formula for the family .     

Quantum inverse scattering method for the q-boson model and symmetric functions

The purpose of this paper is to show that the quantum inverse scattering method for the so-called q-boson model has a nice interpretation in terms of the algebra of symmetric functions. In particular, in the case of the phase model (corresponding to q = 0) the creation operator coincides (modulo a scalar factor) with the operator of multiplication by the generating function of complete homogeneous symmetric functions, and the wave functions are expressed via the Schur functions s_λ(x). The general case of the q-boson model is related in a similar way to the Hall-Littlewood symmetric functions P_λ(x;q²).     

Quantum Jacobi forms and finite evaluations of unimodal rank generating functions

In this paper, we introduce the notion of a quantum Jacobi form, and offer the two-variable combinatorial generating function for ranks of strongly unimodal sequences as an example. We then use its quantum Jacobi properties to establish a new, simpler expression for this function as a two-variable Laurent polynomial when evaluated at pairs of rational numbers. Our results also yield a new expression for radial limits associated to the partition rank and crank functions previously studied by Ono, Rhoades, and Folsom.     

Floquet exponents and stabilizability in time-periodic parabolic systems

We give asymptotic estimates of the Floquet exponents of a class of time-periodic parabolic systems. We apply the result to the problem of stabilizability of such systems.This work was partially supported by the Italian National Project M.P.I 40% Equazioni di Evoluzione e Applicazioni Fisico-Matematiche.     

Floquet theory and stability of nonlinear integro-differential equations

Summary One of the classical topics in the qualitative theory of differential equations is the Floquet theory. It provides a means to represent solutions and helps in particular for stability analysis. In this paper first we shall study Floquet theory for integro-differential equations (IDE), and then employ it to address stability problems for linear and nonlinear equations.     

Floquet theory for the stability of boundary layer flows

A Floquet theory is presented for the stability of boundary layer flows. The spectrum of the governing differential operator is partially discrete and partially continuous, which is different from confined flows. However, no instabilities appear from the continuous part of the spectrum.