A general realization theorem for matrix-valued Herglotz-Nevanlinna functions

Sergey Belyi*, Seppo Hassi, Henk de Snoo, Eduard Tsekanovskii

*Corresponding author for this work

    Research output: Contribution to journalArticleAcademicpeer-review

    8 Citations (Scopus)


    New special types of stationary conservative impedance and scattering systems, the so-called non-canonical systems, involving triplets of Hilbert spaces and projection operators, are considered. It is established that every matrix-valued Herglotz-Nevanlinna function of the form

    V(z) = Q + Lz + integral(R) (1/t-z - t/1 + t(2)) d Sigma(t)

    can be realized as a transfer function of such a new type of conservative impedance system. In this case it is shown that the realization can be chosen such that the main and the projection operators of the realizing system satisfy a certain commutativity condition if and only if L = 0. It is also shown that V(z) with an additional condition (namely, L is invertible or L = 0), can be realized as a linear fractional transformation of the transfer function of a non-canonical scattering F+-system. In particular, this means that every scalar Herglotz-Nevanlinna function can be realized in the above sense. Moreover, the classical Livsic systems (Brodskil-Livsic operator colligations) can be derived from F+-systems as a special case when F+ = I and the spectral measure d Sigma(t) is compactly supported. The realization theorems proved in this paper are strongly connected with, and complement the recent results by Ball and Staffans. (c) 2006 Elsevier Inc. All rights reserved.

    Original languageEnglish
    Pages (from-to)331-358
    Number of pages28
    JournalLinear Algebra and Its Applications
    Issue number2-3
    Publication statusPublished - 1-Dec-2006


    • operator colligation
    • conservative and impedance system
    • transfer (characteristic) function


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