Algorithmic determination of infinite-dimensional symmetry groups for integrable systems in 1 + 1 dimensions

We present algorithms and describe CA-packages to compute the infinitesimal generators of infinite-dimensional symmetry groups for integrable PDEs (evolution equations) in one space and one time dimension. Here, integrable is meant in the sense that the vector field defining the equation is a member of the abelian part of some infinite-dimensional Virasoro algebra. The method of computation is completely different from the usual prolongation method, no determining equations are solved. Instead, all necessary generators of the finitely generated Virasoro algebra are computed from one given element by direct Lie algebra methods. The implementation of the algorithms in MuPAD is described. A sample session is included in which the recursion structures of the KdV and the Krichever-Novikov equations are computed.     

Stability theory for infinite-dimensional linear systems based on an extension method

The state for a homogeneous infinite-dimensional linear systemcan be extended to the dual of a countably Hilbertian spacewith graph topology. The stability theory for the extensionof the system is illustrated. We prove that the exponentialgrowth bound of the extended system is the infimum of suchthat the resolvent is a funtion of finite order in the halfplane Re . Relationships between various notions of stabilityof the original system and of its extension are considered.For the question of exponential stability for physical problem,the original theory may lead to opposite results while the extensionmethod will avoid the contradiction.     

Quantum integrable systems and differential Galois theory

This paper is devoted to a systematic study of quantum completely integrable systems (i.e., complete systems of commuting differential operators) from the point of view of algebraic geometry. We investigate the eigenvalue problem for such systems and the correspondingD-module when the eigenvalues are in generic position. In particular, we show that the differential Galois group of this eigenvalue problem is reductive at generic eigenvalues. This implies that a system is algebraically integrable (i.e., its eigenvalue problem is explicitly solvable in quadratures) if and only if the differential Galois group is commutative for generic eigenvalues. We apply this criterion of algebraic integrability to two examples: finite-zone potentials and the elliptic Calogero-Moser system. In the second example, we obtain a proof of the Chalyh-Veselov conjecture that the Calogero-Moser system with integer parameter is algebraically integrable, using the results of Felder and Varchenko.     

Square integrable holomorphic functions on infinite-dimensional Heisenberg type groups

We introduce a class of non-commutative, complex, infinite-dimensional Heisenberg like Lie groups based on an abstract Wiener space. The holomorphic functions which are also square integrable with respect to a heat kernel measure μ on these groups are studied. In particular, we establish a unitary equivalence between the square integrable holomorphic functions and a certain completion of the universal enveloping algebra of the “Lie algebra” of this class of groups. Using quasi-invariance of the heat kernel measure, we also construct a skeleton map which characterizes globally defined functions from the L ²(ν)-closure of holomorphic polynomials by their values on the Cameron–Martin subgroup.     

A note on generalized invariant subspaces for infinite-dimensional systems

In this paper, some generalized invariant subspaces for uncertainlinear infinite-dimensional systems in the sense that each uncertainparameters are in given real intervals are studied.     

A-posteriori KAM theory with optimal estimates for partially integrable systems

In this paper we present a-posteriori KAM results for existence of d-dimensional isotropic invariant tori for n-DOF Hamiltonian systems with additional $n?d$ independent first integrals in involution. We carry out a covariant formulation that does not require the use of action-angle variables nor symplectic reduction techniques. The main advantage is that we overcome the curse of dimensionality avoiding the practical shortcomings produced by the use of reduced coordinates, which may cause difficulties and underperformance when quantifying the hypotheses of the KAM theorem in such reduced coordinates. The results include ordinary and (generalized) iso-energetic KAM theorems. The approach is suitable to perform numerical computations and computer assisted proofs.     

Velocity inversion: A case study in infinite-dimensional optimization

The goal of seismic velocity inversion is the estimation of seismic wave velocities inside the earth by attempting to predict, in a least-error sense, seismic waveforms measured at its surface. We present velocity inversion as a case study in the various infinite-dimensional pathologies which may afflict practically important problems of distributed parameter identification, treated as optimization problems in function spaces. These features differentiate various problem formulations far beyond the degree one would expect for finite- (small-) dimensional problems. We illustrate this differentiation by comparing the characteristics of three different least-squares formulations of velocity inversion.     

Graded Lie algebras, representation theory, integrable mappings, and integrable systems

A new class of integrable mappings and chains is introduced. The corresponding 1+2 integrable systems that are invariant under such integrable mappings are presented in an explicit form. Soliton-type solutions of these systems are constructed in terms of matrix elements of fundamental representations of semisimple A_n algebras for a given group element. The possibility of generalizing this construction to the multidimensional case is discussed. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 122, No. 2, pp. 251–271, February, 2000.     

Recent advances in the monodromy theory of integrable Hamiltonian systems

The notion of monodromy was introduced by J.J. Duistermaat as the first obstruction to the existence of global action coordinates in integrable Hamiltonian systems. This invariant was extensively studied since then and was shown to be non-trivial in various concrete examples of finite-dimensional integrable systems. The goal of the present paper is to give a brief overview of monodromy and discuss some of its generalizations. In particular, we will discuss the monodromy around a focus–focus singularity and the notions of quantum, fractional and scattering monodromy. The exposition will be complemented with a number of examples and open problems.     

Perturbation theory in a nonselfadjoint special case

Translated fromIssledovaniya po Prikladnoi Matematike, No. 19, 1992, pp. 81–89.     

Non-local Poisson structures and applications to the theory of integrable systems

We develop a rigorous theory of non-local Poisson structures, built on the notion of a non-local Poisson vertex algebra. As an application, we find conditions that guarantee applicability of the Lenard–Magri scheme of integrability to a pair of compatible non-local Poisson structures. We apply this scheme to several such pairs, proving thereby integrability of various evolution equations, as well as hyperbolic equations.     

Remarks on integrable systems

The problem of integrability conditions for systems of differential equations is discussed. Darboux’s classical results on the integrability of linear non-autonomous systems with an incomplete set of particular solutions are generalized. Special attention is paid to linear Hamiltonian systems. The paper discusses the general problem of integrability of the systems of autonomous differential equations in an n-dimensional space, which admit the algebra of symmetry fields of dimension ? n. Using a method due to Liouville, this problem is reduced to investigating the integrability conditions for Hamiltonian systems with Hamiltonians linear in the momenta in phase space of dimension that is twice as large. In conclusion, the integrability of an autonomous system in three-dimensional space with two independent non-trivial symmetry fields is proved. It should be emphasized that no additional conditions are imposed on these fields.     

Perturbation theory for pseudo-inverses

A perturbation theory for pseudo-inverses is developed. The theory is based on a useful decomposition (theorem 2.1) ofB ⁺ -A ⁺ whereB andA arem ×n matrices. Sharp estimates of B ⁺ -A ⁺ are derived for unitary invariant norms whenA andB are of the same rank and B -A is small. Under similar conditions the perturbation of a linear systemAx=b is studied. Realistic bounds on the perturbation ofx=A ⁺ b andr=b=Ax are given. Finally it is seen thatA ⁺ andB ⁺ can be compared if and only ifR(A) andR(B) as well asR(A ^H ) andR(B ^H ) are in the acute case. Some theorems valid only in the acute case are also proved.This work was sponsored in part by The Swedish Institute of Applied Mathematics.     

Parameter-insensitive disturbance rejection for infinite-dimensional systems

In the framework of the so-called geometric approach, this paperstudies two parameter-insensitive disturbance-rejection problemswith state feedback and with incomplete state feedback for linearsystems defined in Hilbert spaces, and presents necessary and/orsufficient conditions for these problems to be solvable undercertain assumptions.     

Non-coercive Lyapunov functions for infinite-dimensional systems

We show that the existence of a non-coercive Lyapunov function is sufficient for uniform global asymptotic stability (UGAS) of infinite-dimensional systems with external disturbances provided the speed of decay is measured in terms of the norm of the state and an additional mild assumption is satisfied. For evolution equations in Banach spaces with Lipschitz continuous nonlinearities these additional assumptions become especially simple. The results encompass some recent results on linear switched systems on Banach spaces. Finally, we derive new non-coercive converse Lyapunov theorems and give some examples showing the necessity of our assumptions.     

A topological property of integrable systems

If we are given real-valued smooth functions on which are in involution, then, under some mild hypotheses, the subset of where these functions are linearly independent is not simply connected.