## All optimal Hankel-norm approximations of linear multivariable systems and their $$L^{\infty}$$-error bounds.(English)Zbl 0543.93036

The problem is considered of approximating a linear system, with transfer function matrix $$G(s)$$ of McMillan degree n, by a linear system with transfer function matrix $$\hat G(s)$$ of McMillan degree $$k<n.$$ A complete characterization is derived of all approximations that minimize the Hankel norm of the error system: $$\| G-\hat G\|_ H.$$ The Hankel norm is an induced operator norm obtained by considering a linear system as a mapping between square integrable ”past” inputs and square integrable ”future” outputs. The key to the solution to this problem is the characterization of all matrices, with rational entries, in the form $$\hat G(s)+F(s)$$ which minimize $\| G-\hat G-F\| =\sup_{\omega}{\bar \sigma}(G(j\omega)-\hat G(j\omega)-F(j\omega)),$ where $$\hat G(s)$$ has McMillan degree k, $$F(s)$$ is anti-causal and $${\bar \sigma}$$(.) denotes the maximum singular value of a matrix. The solution to this problem is obtaind using results on balanced realizations, all- pass functions and the inertia of matrices. An algorithm is presented for finding Hankel norm approximations, and various error bounds on the approximation error are given. For one class of approximants it is shown that $$\| G-\hat G\| \leq \sum^{n}_{i=k+1}\sigma_ i(G),$$ where $$\sigma_ i(G)$$ is the $$i^{th}$$ largest Hankel singular value of G. Bounds such as this are important for predicting the performance of control schemes designed for $$\hat G$$ but applied to G.
Although the impetus for obtaining the results contained in this paper owes much to the pioneering work of V. M. Adamyan, D. Z. Arov and M. G. Krejn [Mat. Sb., Nov. Ser. 86(128), 34-75 (1971; Zbl 0243.47023); Izv. Akad. Nauk Arm. SSR, Mat. 6, 87-112 (1971; Zbl 0311.15012)] on the approximation of Hankel matrices, the author has developed his own rather ingenious methods of obtaining closed form solutions to the finite dimensional case. The paper is selfcontained, and although rather long, is extremely well written and will reward its reader with many interesting results.
Reviewer: D.A.Wilson

### MSC:

 93C35 Multivariable systems, multidimensional control systems 15A60 Norms of matrices, numerical range, applications of functional analysis to matrix theory 41A20 Approximation by rational functions 15B57 Hermitian, skew-Hermitian, and related matrices 93C05 Linear systems in control theory 47A30 Norms (inequalities, more than one norm, etc.) of linear operators 93A15 Large-scale systems

### Citations:

Zbl 0243.47023; Zbl 0311.15012
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### References:

  DOI: 10.1070/SM1971v015n01ABEH001531 · Zbl 0248.47019  ANDERSON , B. D. O. , and LATHAM , G. A. , 1984 , report under preparation .  ANDERSON B. D. O., Network Analysis and Synthesis A Modem Systems Theory Approach (1973)  BARNETT S., Matrices in Control Theory with Applications to Linear Programming (1971) · Zbl 0245.93002  BARTELS R. H., Communs. Ass. comput. Mach 415 pp 820– (1972)  BULTHEEL A., I.E.E.E. Trans. Circuits Syst  BETTAYEB , M. , SILVERMAN , L. M. , and SAFONOV , M. G. , 1980 ,Proc. I.E.E.E. Conj. on Decision and Control, Albuquerque , New Mexico , p. 195 .  DOI: 10.1007/BFb0065310  CURTAIN , R. F. , and GLOVER , K. , 1984 , report under preparation .  DUREN P. L., Theory of HP Spaces (1970) · Zbl 0215.20203  DOI: 10.1007/BF02288367 · JFM 62.1075.02  GANTMACHER F. R., The Theory of Matrices 11 (1966) · Zbl 0136.00410  GLOVER , K. , 1984 , report under preparation .  GLOVER K., I.E.E.E. Trans.autom. Control (1983)  HAMMARLINO S. J., I.M.A.J. of Numer. Anal 2 pp 303– (1982)  HILDEBRAND K., Introduction to Numerical Analysis (1956) · Zbl 0070.12401  HOFFMAN K., Banach Spaces of Analytic Functions (1962) · Zbl 0117.34001  JONCKHEERE , E. A. , SAFONOV , M. G. , and SILVERMAN , L. M. , 1981 ,Proc. 20thI.E.E.E. Conf. on Decision and Control, p. 118 .  KAILATH T., Linear Systems (1980)  DOI: 10.1109/TAC.1981.1102736 · Zbl 0553.93038  LAUB A. J., Joint Automatic Control Conf (1980)  DOI: 10.1007/BF01599021 · Zbl 0512.93008  LUENBERGER D. G., Optimization by Vector Space Methods (1969) · Zbl 0176.12701  DOI: 10.1093/qmath/11.1.50 · Zbl 0105.01101  DOI: 10.1109/TAC.1981.1102568 · Zbl 0464.93022  DOI: 10.2307/1969670 · Zbl 0077.10605  DOI: 10.1016/0022-247X(62)90030-6 · Zbl 0112.01401  DOI: 10.1109/TAC.1982.1102945 · Zbl 0482.93024  DE PRONY R., J. Ec polytech. (Paris) 1 pp 24– (1795)  RUDIN W., Real and Complex Analysis (1966) · Zbl 0142.01701  SAFONOV M. G., Proc. American Control Conf (1983)  SARASON D., Am. Math. Soc. Transl 127 pp 180– (1967)  SILVERMAN L. M., Proc. Joint Automatic Control Conf (1980)  STEWART G. W., Introduction to Matrix Computation (1973) · Zbl 0302.65021  DOI: 10.1016/0024-3795(74)90060-3 · Zbl 0288.15015  DOI: 10.1109/TIT.1961.1057636
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