Hessenberg pairs of linear transformations

Let denote a field and V denote a nonzero finite-dimensional vector space over . We consider an ordered pair of linear transformations A:V→V and A^*:V→V that satisfy (i)–(iii) below.

1. [(i)]Each of A,A^* is diagonalizable on V.

2. [(ii)]There exists an ordering of the eigenspaces of A such that

where V_-1=0, V_d+1=0.

3. [(iii)]There exists an ordering of the eigenspaces of A^* such that

where , .

We call such a pair a Hessenberg pair on V. In this paper we obtain some characterizations of Hessenberg pairs. We also explain how Hessenberg pairs are related to tridiagonal pairs.

On the pointwise ergodic behaviour of transformations preserving infinite measures

We consider situations in which the asymptotic type of a measure preserving transformation manifests itself in a pointwise manner.     

Extensions of pairs of linear transformations between infinite-dimensional vector spaces

To a pair A, B:V→W of linear maps between complex vector spaces attach the pair (V, W) endowed with the operation (α, β)υ = (αA + βB)(υ), α,β ∈ C, υ ∈ V. A concept of rank, similar to the torsion-free rank of abelian groups, is definable for the systems (V, W). With appropriate morphisms, the systems from an abelian category and Ext¹ can be construed as a vector space valued functor. We find all the cases in which Ext¹ ((V, W), (X, Y)), with (X, Y), (V, W) indecomposable systems of rank 0 or 1, is finite-dimensional, and compute its dimension in these cases. This extends a former computation for finite-dimensional systems.     

On orbits of fractional linear transformations

Let K be a field and let σ(t) be an automorphism of k(t) of infinite order. In his paper [1] Gerard Cauchon asked whether the transformation which is naturally defined on kU∞ by σ has infinitely many infinite orbits. This is indeed so.     

On birational transformations of pairs in the complex plane

This article deals with the study of the birational transformations of the projective complex plane which leave invariant an irreducible algebraic curve. We try to describe the state of the art and provide some new results on this subject.      

On the number of idempotent linear transformations

Let M and N be two subspaces of a finite dimensional vector space V over a finite field F. We can count the number of all idempotent linear transformations T of V such that R(T) ?M and N?N(T), where R(T) and N(T) denote the range space and the null space of T, respectively.     

Some new transformations for Bailey pairs and WP-Bailey pairs

We derive several new transformations relating WP-Bailey pairs. We also consider the corresponding transformations relating standard Bailey pairs, and as a consequence, derive some quite general expansions for products of theta functions which can also be expressed as certain types of Lambert series.     

On complete controllability of linear nonstationary systems

By use of the first integrals of the equations of unperturbed motion we obtain necessary and sufficient conditions for complete controllability of linear nonstationary dynamical systems.Translated fromMatematicheskie Metody i Fiziko-Mekhanicheskie Polya, Issue 35, 1992, pp. 160–163.     

On linear transformations preserving at least one eigenvalue

Let be an algebraically closed field and be a linear transformation. In this paper we show that if preserves at least one eigenvalue of each matrix, then preserves all eigenvalues of each matrix. Moreover, for any infinite field (not necessarily algebraically closed) we prove that if is a linear transformation and for any with at least an eigenvalue in , and have at least one common eigenvalue in , then preserves the characteristic polynomial.

     

On rational transformations of linear functionals: direct problem

Let u be a quasi-definite linear functional. We find necessary and sufficient conditions in order to the linear functional v satisfying be a quasi-definite one. Also we analyze some linear relations linking the polynomials orthogonal with respect to u and v.     

Reflexive linear transformations

A linear operator A is called reflexive if the only operators that leave invariant the invariant subspaces of A are the operators in the weak closure of the algebra of polynomials in A. In this note we completely characterize reflexive operators on finite-dimensional spaces.     

On common linear copositive Lyapunov functions for pairs of stable positive linear systems

We study the common linear copositive Lyapunov functions of positive linear systems. Firstly, we present a theorem on pairs of second order positive linear systems, and give another proof of this theorem by means of properties of geometry. Based on the process of the proof, we extended the results to a finite number of second order positive linear systems. Then we extend this result to third order systems. Finally, for higher order systems, we give some results on common linear copositive Lyapunov functions.     

On some pointwise inequalities

In a recent paper (Studia Math. 138 (2000) 285-291) we proved pointwise estimates relating some classical maximal and singular integral operators. Here we show that, in a sense, there are more flexible inequalities which not only imply the previously known results but also give something new. In particular, they hold for the multilinear Calderón-Zygmund operators. This result gives a new approach to a recent work by Grafakos and Torres, and unifies some classical inequalities by Cotlar and Coifman and Fefferman.     

On the complete controllability of linear nonautonomous systems

We suggest a geometric approach to the controllability of nonautonomous linear control systems of the form $\dot x = A(t)x + B(t)u$ , x ?? ? ⁿ , u ?? U ? ? ^m , with conical control constraint set U and continuous matrices A(t) and B(t). We derive two new complete controllability criteria, the first of which is reduced to the analysis of the arrangement of the cones ??^?1(t)B(t)U in the state space of the system [ $\dot \Phi (t) = \left. {A(t)\Phi } \right|(t)$ , ??(0) = E] and the second is based on the existence of appropriate controls bringing zero back to zero. We prove a theorem on the approximation of the control constraint set U by cones with finitely many generators lying inside the cone U with the preservation of the complete controllability property. We present a number of examples illustrating some peculiarities in the evolution of controllability sets of nonautonomous linear systems.