Strong solutions of the discrete-time algebraic Riccati equation

Conditions are given under which a solution of the DARE is positive semidefinite if and only if all the eigenvalues of its associated closed-loop matrix are in the closed unit disc.     

Existence and uniqueness of unmixed solutions of the discrete-time algebraic Riccati equation

A solution X of a discrete-time algebraic Riccati equation is called unmixed if the corresponding closed-loop matrix Φ(X) has the property that the common roots of det(sI−Φ(X)) and det(I−sΦ(X)^*) (if any) are on the unit circle. A necessary and sufficient condition is given for existence and uniqueness of an unmixed solution such that the eigenvalues of Φ(X) lie in a prescribed subset of .     

The dual algebraic Riccati equations and the set of all solutions of the discrete-time Riccati equation

In this paper, two new pairs of dual continuous-time algebraic Riccati equations (CAREs) and dual discrete-time algebraic Riccati equations (DAREs) are proposed. The dual DAREs are first studied with some nonsingularity assumptions on the system matrix and the parameter matrix. Then, in the case of singular matrices, a generalised inverse is introduced to deal with the dual DARE problem. These dual AREs can easily lead us to an iterative procedure for finding the anti-stabilising solutions, especially to DARE, by means of that for the stabilising solutions. Furthermore, we provide the counterpart results on the set of all solutions to DARE inspired by the results for CARE. Two examples are presented to illustrate the theoretical results.     

Algorithm for solving algebraic Riccati equation which has singular Hamiltonian matrix

The algorithm of construction of the solution of ARE, with Hamiltonian matrix having zero eigenvalues, is developed. The algorithm generalizes the Schur method on ARE with singular Hamiltonian matrix and could be used for J-factorization of matrix polynomial, which has zero roots.     

On the existence of solutions of the general algebraic Riccati equation

This paper addresses the problem of real symmetric solutions of the general algebraic Riccati equation (GARE) with an indefinite quadratic term. The GARE arises in linear quadratic differential games, in the stabilization of uncertain systems, robust optimal control and disturbance attenuation problems. Using the properties of the solutions of the differential equation corresponding to the GARE and the related conclusions of differential games, we have established the main results of this paper. Theorems 1 and 2     

Comments on "On the numerical solution of the discrete-time algebraic Riccati equation"

The problem discussed in the above paper has been independently considered by the authors in [1], [2]. The purpose of this note is to point out the differences with our approach of solving the same problem, to present a solution to the eigenvalue reordering problem which is left as work to be done in the paper, and to present an extension suitable for problems with singular loss matrix on control.     

Convergence of the doubling algorithm for the discrete-time algebraic Riccati equation

If the doubling algorithm (DA) for the discrete-time algebraic Riccati equation converges, the speed of convergence is high. However, its convergence has not yet been examined exactly. Firstly, a certain matrix appearing in the DA is shown to be non-singular and therefore the algorithm is well defined. Secondly, it is found that the loss of significant digits hardly occurs in the DA. Finally, it is proved that if the time-invariant discrete-time linear system, whose state is estimated by the steady-state Kalman filter, is reachable and detectable, or stabilizable and observable, then all three matrix sequences in the DA converge.     

Hermitian solutions of the discrete algebraic Riccati equation

Hermitian solutions of the discrete algebraic Riccati equation play an important role in the least-squares optimal control problem for discrete linear systems. In this paper we describe the set of hermitian solutions in various ways: in terms of factorizations of rational matrix functions which take hermitian values on the unit circle; in terms of certain invariant subspaces of a matrix which is unitary in an indefinite scalar product; and in terms of all invariant subspaces of a certain matrix. These results are inspired by known results for the algebraic Riccati equation arising in the least-squares optimal control problem for continuous linear systems.     

Existence of unmixed solutions of the discrete-time algebraic Riccati equation and a nonstrictly bounded real lemma

The existence of a solution of the discrete-time algebraic Riccati equation is established assuming modulus controllability and positive semidefiniteness on the unit circle of the Popov function. As an application a nonstrictly bounded real lemma is obtained.     

On the discrete time algebraic Riccati equation

A detailed account of the properties of a class of algebraic Riccati equations which arise in discrete time control and filtering problems is given. It is shown that a generalized notion of detectability plays an important role in classifying solutions of these equations. This concept is also related to a minimum phase condition.     

On closed form solutions for the differential matrix Riccati equation problem

A new formulation of the differential matrix Riccati equation is presented and a closed analytical solution is obtained under the hypothesis that certain commutativity conditions are fulfilled on a transformed space. The formulation generalizes the results of [1] on algebraic equations to differential matrix Riccati equations. To illustrate the usefulness of the method, a closed analytical solution of the differential matrix Riccati equation is obtained inR^{2 times 2}.     

Scaling of the discrete-time algebraic Riccati equation to enhancestability of the Schur solution method

A simple scaling procedure for discrete-time Riccati equations is introduced. This procedure eliminates instabilities which can be associated with the linear equation solution step of the generalized Schur method without changing the condition of the underlying problem. A computable bound for the relative error of the solution of the Riccati equation is also derived     

Lower solution bounds of the continuous coupled algebraic Riccati matrix equation

In this paper, applying eigenvalue sum inequality of symmetric matrix and the properties of M-matrix and its inverse matrix, we introduce new lower matrix bounds for the solution of the continuous coupled algebraic Riccati equation. Finally, we give corresponding numerical examples to demonstrate the effectiveness of the derived results.     

New upper solution bounds of the continuous algebraic Riccati matrix equation

By utilizing some linear algebraic techniques, new upper bounds of the solution of the continuous algebraic Riccati equation (CARE) are derived. According to the present bounds, iterative procedures are also developed for obtaining more precise estimations. Comparing with existing results, the obtained bounds are less restrictive.     

Employing the algebraic Riccati equation for a parametrization of the solutions of the finite-horizon LQ problem: the discrete-time case

In this paper, a new methodology is developed for the closed-form solution of a generalized version of the finite-horizon linear-quadratic regulator problem for LTI discrete-time systems. The problem considered herein encompasses the classical version of the LQ problem with assigned initial state and weighted terminal state, as well as the so-called fixed-end point version, in which both the initial and the terminal states are sharply assigned. The present approach is based on a parametrization of all the solutions of the extended symplectic system. In this way, closed-form expressions for the optimal state trajectory and control law may be determined in terms of the boundary conditions. By taking advantage of standard software routines for the solution of the algebraic Riccati and Stein equations, our results lead to a simple and computationally attractive approach for the solution of the considered optimal control problem without the need of iterating the Riccati difference equation.     

On upper bounds for the unified algebraic Riccati equation

In recent years, several eigenvalues, norms and determinants bounds have been investigated separately for the solutions of continuous and discrete Riccati equations. In this paper, an upper bound for solution of the unified Riccati equation is presented. In the limiting cases, the result reduces to a new upper bound for the solution of continuous and discrete Riccati equation.     

Bounds for the eigenvalues of the solution matrix of the algebraic Riccati equation

Novel bounds are proposed for the extreme and lower half eigenvalues of the solution matrix for the algebraic Riccati equation. The formulae giving these bounds can easily be applied to determine the region where the eigenvalues lie, and the bounds have the added advantage of being sharper in some cases than the previously proposed ones, as some realistic examples will show. The proposed bounds find many applications which are pointed out in the text.     

New upper matrix bounds of the solution for perturbed continuous coupled algebraic Riccati matrix equation

In this paper, if the coefficient matrices in the continuous coupled algebraic Riccati equation (CCARE) undergo perturbations, with the aid of the equivalent form for the perturbation of the CCARE and the classical eigenvalue inequalities, we observe new upper matrix bounds for the perturbation of the CCARE through solving the linear inequalities. Finally, we present corresponding numerical examples to show the effectiveness of the derived results.     

Curvature properties of the algebraic Riccati equation

We prove that the solution to the algebraic Ricatti equation (ARE) is concave with respect to a nonnegative-definite symmetric state weighting matrix Q when the input weighting matrix R = R^T > 0. We also prove that the solution to the ARE is concave with respect to a positive-definite diagonal input weighting matrix R when Q = Q^T ≥ 0.     

An algorithm using the HR process for solving the closed-loopeigenvalues of a discrete-time algebraic Riccati equation

Presents a numerical approach to the closed-loop spectrum of a discrete-time algebraic Riccati equation. The concerned symplectic pencil N-λL is proven to be equivalent to the pencil P-λQ, where P is skew-symmetric and Q is symmetric. Then the HR process, which can be viewed as a generalization of the QR method, is applied to compute the eigenvalue of P-λQ. Some numerical examples are included