rexpi

by Tobias Jawecki

A Python package providing algorithms to compute unitary $(n,n)$-rational best approximations

$$r(\mathrm{i} x) \approx \mathrm{e}^{\mathrm{i}\omega x},\qquad\text{for $x\in[-1,1]$ and a frequency $\omega>0$}.$$

Unitary $(n,n)$-rational functions correspond to rational functions $r=p/q$ where $p$ and $q$ are polynomials of degree $\leq n$ with

$$|r(\mathrm{i} x)|=1,\qquad \text{for $x\in\mathbb{R}$}.$$

Our focus is best approximations in a Chebyshev sense which minimize the uniform error

$$\max_{x\in[-1,1]}| r(\mathrm{i} x) - \mathrm{e}^{\mathrm{i}\omega x} |.$$

Besides new code, this package also contains some routines of the baryrat package¹, and variants of the AAA-Lawson²³ method which is part of the Chebfun package⁴.

This package is still under development. Future versions might have different API. The current version is not fully documented.

Geting started

Install the package python -m pip install rexpi and run numerical examples from the /examples folder.

Best approximations and equioscillating phase errors

Following a recent work⁵, for $\omega\in(0,\pi(n+1))$ the unitary best approximation to $\mathrm{e}^{\mathrm{i}\omega x}$ is uniquely characterized by an equioscillating phase error. In particular, unitary rational functions are of the form $r(\mathrm{i} x) = \mathrm{e}^{\mathrm{i}g(x)}$ for $x\in\mathbb{R}$ where $g$ is a phase function, and $g(x) - \omega x$ is the phase error of $r(\mathrm{i} x) \approx \mathrm{e}^{\mathrm{i}\omega x}$. The phase error equioscillates at $2n+2$ nodes $\eta_1< \eta_2< \ldots <\eta_{2n+2}$ if $g(\eta_j) - \omega \eta_j = (-1)^{j+1} \max_{x\in[-1,1]}| g(x) - \omega x |$ for $j=1,\ldots,2n+2$.

E.g. the phase and approximation error of the unitary rational best approximation for $n=4$ and $\omega=2.65$ computed by the brib algorithm.

Content

• The best rational interpolation based brib algorithm. This is a modified BRASIL algorithm to compute unitary best approximations to $\mathrm{e}^{\mathrm{i}\omega x}$. r,_,_ = brib(w, n) computes an $(n,n)$-rational approximation to $\mathrm{e}^{\mathrm{i}\omega x}$ for a given frequency $\omega$ and degree $n$.

import numpy as np
import matplotlib.pyplot as plt
import rexpi
n = 10
w = 10
r, _, _ = rexpi.brib(w,n)

# plot the error
xs = np.linspace(-1,1,5000)
err = r(1j*xs)-np.exp(1j*w*xs)
errmax = np.max(np.abs(err))
plt.plot(xs,np.abs(err),[-1,1],[errmax,errmax],':')

• linearizedLawson: A version of the AAA-Lawson method to approximate $\mathrm{e}^{\mathrm{i}\omega x}$ using Chebyshev nodes as pre-assigned support nodes. The approximation computed by the linearizedLawson algorithm is comparable with the approximation computed by the brib algorithm.

n = 10
w = 10
r, _ = rexpi.linearizedLawson(w=w, n=n, nlawson=100,nx=1000)

• Error estimation. Best approximations uniquely exist for $\omega\in(0,\pi(n+1))$ and algorithms may fail when choosing $\omega >\pi(n+1)$. In addition, algorithms may fail if $\omega\approx 0$ due to limits of double precision arithmetic. We suggest using the routine buerrest to determine $\omega$ s.t. the approximation error is bounded by a given tolerance, $\mathrm{tol}>10^{-16}$.

n = 10
tol = 1e-6
w = rexpi.buerrest_getw(n, tol)
r, _, _ = rexpi.brib(w,n)

• The brib and linearizedLawson algorithms also utilize ideas presented in⁶ to preserve unitarity in computer arithmetic and to reduce computational cost. In particular, when computing approximations based on interpolation or on minimizing a linearized error.
• Applications of unitary best approximations to $\mathrm{e}^{\mathrm{i}\omega x}$ are approximations to the scalar exponential function and approximations to exponentials of skew-Hermitian matrices or the action of such matrix exponentials for numerical time integration, see examples/ex5_matrixexponential.ipynb.

Citing rexpi

A full documentation of this package is currently not available. If you use unitary rational best approximations for your scientific publication, please cite T. Jawecki and P. Singh. Unitary rational best approximations to the exponential function, 2023. preprint at https://arxiv.org/abs/2312.13809.

@unpublished{JS23a,
  author = {Jawecki, T. and Singh, P.},
  title = {Unitary rational best approximations to the exponential function},
  eprint = {2312.13809},
  year = 2023,
  note = {preprint at https://arxiv.org/abs/2312.13809}
}

Footnotes

1. C. Hofreither. An algorithm for best rational approximation based on barycentric rational interpolation. Numer. Algorithms, 88(1):365–388, 2021. doi:10.1007/s11075-020-01042-0. ↩

2. Y. Nakatsukasa, O. Sète, and L.N. Trefethen. The AAA algorithm for rational approximation. SIAM J. Sci. Comput., 40(3):A1494–A1522, 2018. doi:10.1137/16M1106122. ↩

3. Y. Nakatsukasa and L.N. Trefethen. An algorithm for real and complex rational minimax approximation. SIAM J. Sci. Comput., 42(5):A3157–A3179, 2020. doi:10.1137/19M1281897. ↩

4. T.A. Driscoll, N. Hale, and L.N. Trefethen, editors. Chebfun Guide. Pafnuty Publications, Oxford, 2014. also available online from https://www.chebfun.org. ↩

5. T. Jawecki and P. Singh. Unitary rational best approximations to the exponential function. to be published. preprint available at https://arxiv.org/abs/2312.13809. ↩

6. T. Jawecki and P. Singh. Unitarity of some barycentric rational approximants. IMA J. Numer. Anal., 2023. doi:10.1093/imanum/drad066. ↩