Abstract
We investigate the problem of approximating the matrix function f(A) by r(A), with f a Markov function, r a rational interpolant of f, and A a symmetric Toeplitz matrix. In a first step, we obtain a new upper bound for the relative interpolation error 1 − r/f on the spectral interval of A. By minimizing this upper bound over all interpolation points, we obtain a new, simple, and sharp a priori bound for the relative interpolation error. We then consider three different approaches of representing and computing the rational interpolant r. Theoretical and numerical evidence is given that any of these methods for a scalar argument allows to achieve high precision, even in the presence of finite precision arithmetic. We finally investigate the problem of efficiently evaluating r(A), where it turns out that the relative error for a matrix argument is only small if we use a partial fraction decomposition for r following Antoulas and Mayo. An important role is played by a new stopping criterion which ensures to automatically find the degree of r leading to a small error, even in presence of finite precision arithmetic.
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Notes
This includes the limiting case \(\frac {d\nu }{dx}(x) = \frac {1}{\pi \sqrt {z-\beta }}\) and \(f^{[\nu ]}(z)=1/\sqrt {z-\beta }\) for \(\alpha \to -\infty \).
In most papers in the literature, the authors estimate the absolute interpolation error. However, by considering the relative error, we may monitor also the error of rational approximants of functions f being a product of a Markov function and a rational function such as the logarithm or the square root, see Examples 5.1 and 5.3.
If \(\mathbb E\) is a finite union of closed intervals, it is sufficient to suppose that interpolation points in \(Int(\mathbb E)\) only have even multiplicity, and there is an even number of interpolation points in any subinterval of \(\mathbb R \setminus Int(\mathbb E)\).
In other approaches like in [55, Section 6.1], the authors eliminate the term 1/(z − x) in the integral leading to bounds for the absolute error.
The interested reader may observe that we do not impose a normalization condition, and hence neither T nor φ is unique, though the function G2m can be shown to be unique, see also Remark 2.3.
However, the underlying rectangular Cauchy matrix \((\frac {1}{z_{j}-x_{k}})\) might be quite ill-conditioned, even after row or column scaling [12, Cor 4.2]. A closer analysis seems to show that the given bound for the inverse condition number is related to the rate of best rational approximants of our f on the interval [c,d].
Without this positivity assumption, [27, Theorem 4.1] observed with (3.8) that, up to \(\mathcal O(\varepsilon )\), we have that \(|\widetilde R_{M}^{(j)}(z_{k}) -\widetilde f^{(j)}_{j}|\approx |\widetilde f^{(j)}_{k} -\widetilde f^{(j)}_{j}| \leq |\widetilde f^{(j)}_{k}| + |\widetilde f^{(j)}_{j}| \leq 2 |\widetilde f^{(j)}_{k}|\approx 2 |\widetilde R_{M}^{(j)}(z_{k})|\), leading to some exponentially increasing growth factor.
Further explanations on Fig. 3 are given in Example 5.2 below.
In the original formulation [16], Denman and Beavers gave a coupled two-term recurrence for Xk and Yk := A− 1Xk. The product form was obtained later in [14], where Yk is replaced by Mk = XkYk. These authors suggest the recurrence \(X_{k+1}=\frac {1}{2} \mu _{k} X_{k}(I + \mu _{k}^{-2} M_{k}^{-1})\), but our recurrence seems to be more suitable in case where the matrices do no longer commute due to finite precision arithmetic. It seems that this slight modification has no impact on the analysis of stability and limiting accuracy, and even hardly no impact on the (relative) error.
It is interesting to compare our findings to those in [35] where the authors approach Aγ after scaling by evaluating Padé approximants at the single interpolation point z1 = ... = z2m = 1 using Stieltjes continued fractions, somehow a confluent counterpart of our approach.
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Acknowledgements
The authors want to thank Stefan Güttel, Marcel Schweitzer, and Leonid Knizhnerman for carefully reading a draft of this manuscript, and for their useful comments.
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Partial financial support was received from the Labex CEMPI (ANR-11-LABX-0007-01).
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Beckermann, B., Bisch, J. & Luce, R. On the rational approximation of Markov functions, with applications to the computation of Markov functions of Toeplitz matrices. Numer Algor 91, 109–144 (2022). https://doi.org/10.1007/s11075-022-01256-4
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DOI: https://doi.org/10.1007/s11075-022-01256-4
Keywords
- Matrix function
- Toeplitz matrices
- Markov function
- Rational interpolation
- Positive Thiele continued fractions