## Berichte der Arbeitsgruppe Technomathematik (AGTM Report)

84

We consider a transmission boundary-value problem for the time-harmonic Maxwell equations neglecting displacement currents. The usual transmission conditions, which require the continuity of the tangential components of the electric and magnetic fields across boundaries are slightly modified. For this new problem we show that the uniqueness of the solution depends on the topological properties of the domains under consideration. Finally we obtain existence results by using a boundary integral equation approach.

85

We consider a transmission boundary-value problem for the time-harmonic Maxwell equations without displacement currents. As transmission conditions we use the continuity of the tangential parts of the magnetic field H and the continuity of the normal components of the magnetization B=müH. This problem, which is posed over all IR3, is then restricted to a bounded domain by introducing artificial boundary conditions. We present uniqueness and existence proofs for this problem using an integral equation approach and compare the results with those obtained in the unbounded case.

86

We consider two transmission boundary-value problems for the time-harmonic Maxwell equations without displacement currents. For the first problem we use the continuity of the tangential parts of the electric and magnetic fields across material discontinuities as transmission conditions. In the second case the continuity of the tangential components of the electric field E is replaced by the continuity of the normal component of the magnetization B=müH. For this problem existence of solutions is already shown in [6]. If the domains under consideration are not simply connected the solution is not unique. In this paper, we improve the regularity results obtained in [6] and then prove existence and uniqueness theorems for the first problem by extracting its solution out of the set of all solutions of the second problem. Thus we establish a connection between the solutions corresponding to the different transmission boundary conditions.

75

In this paper noises and disturbances are treated as distributions of some general class. The problem of sensitivity minimization is considered. A design procedure for the construction of Luenberger observers which estimate the state of a system with a given rate of accuracy has been proposed. The design procedure is applied to identify the first derivatives of an oscillating signal. The constraints on a noise and on a sampling which are necessary to estimate the derivatives to a given accuracy have been obtained.

76

77

A multiparameter, polynomial feedback strategy is introduced to solve the universal adapative tracking problem for a class of multivariable minimum phase system and reference signals generated by a known linear time-invariant differential equation. For 2-input, 2-output, minimum phase systems (A,B,C) with det(CB)0, a different polynomial tracking controller is given which does not invoke a spectrum unmixing set.

78

Several topological necessary conditions of smooth stabilization in the large have been obtained. In particular, if a smooth single-input nonlinear system is smoothly stabilizable in the large at some point of a connected component of equilibria set, then the connected component is to be an unknoted, unbounded curve.

79

Given a proper antistable rational transfer function g, a balanced realization of g is contructed as a matrix representation of the abstract shift realization introduced in Fuhrmann [1976]. The required basis is constructed as a union of sets of polynomials orthogonal with respect to weights given by the square of the absolute values of minimal degree Schmidt vectors of the corresponding Hankel operators. This extends results of Fuhrmann [1991], obtained in the generic case.

80

81

The polynomial approach introduced in Fuhrmann [1991] is extended to cover the crucial area of AAK theory, namely the characterization of zero location of the Schmidt vectors of the Hankel operators. This is done using the duality theory developed in that paper but with a twist. First we get the standard, lower bound, estimates on the number of unstable zeroes of the minimal degree Schmidt vectors of the Hankel operator. In the case of the Schmidt vector corresponding to the smallest singular the lower bound is in fact achieved. This leads to a solution of a Bezout equation. We use this Bezout equation to introduce another Hankel operator which have singular values that are the inverse of the singular values of the original Hankel operator.