# Computing with rational functions

### Mario Berljafa and Stefan GüttelDownload PDF or m-file

## Contents

## Introduction

This toolbox comes with an implementation of a class called `rkfun`, which is the fundamental data type to represent and work with rational functions. Objects of this class are produced by the `rkfit` function (described in [2,3]), which in its simplest use case attemts to find a rational function such that

where are square matrices and is a vector of compatible sizes. For example, let us consider , , and . We know that is a minimizer for the above problem. Let us try to find it via RKFIT:

N = 100; A = gallery('tridiag', N); I = speye(N); F = (A - 4*I)\(A - 2*I)^2/(A^2 - I); b = eye(N,1); xi = inf(1, 5); [xi,ratfun,misfit] = rkfit(F, A, b, xi, 5, 1e-10, 'real');

As we started with initial poles (at infinity), RKFIT will search for a rational function of type . When more than 1 iteration is performed and the tolerance `tol` is not chosen too small, RKFIT will try to reduce the type of the rational function while still maintaining a relative misfit below the tolerance, i.e., . Indeed, a reduction has taken place and the type has been reduced to , as can be seen from the following output:

disp(ratfun)

RKFUN object of type (2, 3). Real-valued Hessenberg pencil (H, K) of size 4-by-3. coeffs = [-0.087, -0.394, -1.331, -1.503]

We can now perform various operations on the `ratfun` object, all implemented as methods of the class `rkfun`. To see a list of all methods just type `help rkfun`. We will now discuss some methods in more detail.

## Evaluating an `rkfun`

We can easily evaluate at any point (or many points) in the complex plane. The following command will evaluate and simultaneously:

disp(ratfun([2; 3+1i]))

-1.2226e-14 + 0.0000e+00i 1.1765e-02 - 1.5294e-01i

We can also evaluate as a matrix function, i.e., computing for matrices and , by using two input arguments. For example, by setting we effectively compute the full matrix function :

M = [3, 1; 0, 3]; B = eye(2); R = ratfun(M, B)

R = -1.2500e-01 -2.8125e-01 0 -1.2500e-01

## Plotting

As can be evaluated at any point in the complex plane, it is straightforward to produce plots of this function. For example, here is a contour plot of over the complex region :

```
figure(1)
[X,Y] = meshgrid(-2:.01:5, -1:.01:1);
Z = X + 1i*Y;
R = ratfun(Z);
contourf(X, Y, log10(abs(R)), -4:.25:2)
colormap hot, colorbar
```

The following plot shows the phase portrait of on the same domain. Can you spot the three poles and a double root?

```
figure(3)
contourf(X, Y, angle(R), 20)
set(gca,'CLim',[-pi,+pi])
colormap(util_colormapc(1, 90));
colorbar
```

Another command `ezplot` can be used to get a quick idea of how `ratfun` looks over an interval on the real axis, in this case, :

```
figure(2)
ezplot(ratfun, [-2, 5])
ylim([-5, 5])
grid on
```

## Pole- and root-finding

From the above plot we guess that has poles at and and a root at , which is to be expected from the definition of . The two commands `poles` and `roots` do exactly what their names suggest:

pls = poles(ratfun) rts = roots(ratfun)

pls = -1.0000e+00 1.0000e+00 4.0000e+00 rts = 2.0000e+00 + 2.7597e-07i 2.0000e+00 - 2.7597e-07i

As expected from a type (2,3) rational function, there are two roots and three poles. Note that the pole at is identified with slightly less accuracy than the poles at and . This is because the point is outside the spectral interval of and hence sampled less accurately. Also the double root at is identified up to an accuracy of only. This is not surprising as the function is flat nearby multiple roots. However, the backward error of the roots is small:

disp(ratfun(rts))

4.9960e-16 + 1.0588e-22i 4.9960e-16 - 1.0588e-22i

## Basic arithmetic operations and differentiation

We have implemented some very basic operations for the `rkfun` class, namely, the multiplication by a scalar and the addition of a scalar. The result of such operations is again an `rkfun` object. For example, the following command computes points where :

z = roots(2*ratfun - pi) disp(ratfun(z))

z = -4.0504e-01 8.5769e-01 1.5708e+00 1.5708e+00

We have currently not implemented the summation and multiplication of two `rkfun` objects, though this is doable in principle. **---UPDATE: This is no longer the case since Version 2.2 of the toolbox. Check out the example on "Electronic filter design using RKFUN arithmetic".---** However, we can already differentiate a rational function using the `diff` command. The following will find all real local extrema of by computing the real roots of :

```
extrema = roots(diff(ratfun),'real')
```

extrema = 4.0759e-01 2.0000e+00

There are two real extrema which we can add to the above plot of :

figure(2), hold on plot(extrema, ratfun(extrema), 'ro')

The syntax and "feel" of these computations is inspired by the Chebfun system [4], which represents polynomials via Chebyshev interpolants and allows for many more operations to be performed than our `rkfun` implementation. Here we are representing rational functions through their coefficients in a discrete-orthogonal rational function basis. Working with rational functions poses some challenges not encountered with polynomials. For example, the indefinite integral of a rational function is not necessarily a rational function but may contain logarithmic terms.

## Multiple precision computations

Objects of class `rkfun` can be converted to MATLAB's Variable Precision Arithmetic (VPA) as follows:

disp(vpa(ratfun))

RKFUN object of type (2, 3). Real-valued Hessenberg pencil (H, K) of size 4-by-3. Variable precision arithmetic (VPA) activated. coeffs = [-0.087, -0.394, -1.331, -1.503]

Alternatively, we can also use the Advanpix Multiple Precision (MP) toolbox [1], which is typically more efficient and reliable than VPA. We recommend the use of this toolbox in particular for high-precision root-finding of `rkfun`'s and conversion to partial fraction form:

ratfun = mp(ratfun)

ratfun = RKFUN object of type (2, 3). Real-valued Hessenberg pencil (H, K) of size 4-by-3. Multiple precision arithmetic (ADVANPIX) activated. coeffs = [-0.087, -0.394, -1.331, -1.503]

When evaluating a multiple precision ratfun, the result will be returned in multiple precision:

```
format longe
ratfun(2)
```

ans = -1.2125294919816719385880796519441591e-14

It is important to understand that, although the evaluation of `ratfun` is now done in multiple precision, the computation of `ratfun` using the `rkfit` command has been performed in standard double precision. `rkfit` does not currently support the computation of `rkfun` objects in multiple precision. Here are the roots of `ratfun` computed in multiple precision:

roots(ratfun)

ans = Columns 1 through 1 2.00000000000005884325215506037004691e+00 + 2.69725359428640320019961191862469502e-07i 2.00000000000005884325215506037004691e+00 - 2.69725359428640320019961191862469502e-07i

## Conversion to partial fraction form

It is often convenient to convert a rational function into its partial fraction form

The `residue` command of our toolbox performs such a conversion. Currently, this only works when the poles of are distinct and is not of superdiagonal type (i.e., there is no linear term in ). As the conversion to partial fraction form can be an ill-conditioned transformation, we recommend to use `residue` in conjunction with the multiple precision feature. Here are the poles and residues (), as well as the absolute term , of the function defined above:

[alpha, xi, alpha0, cnd] = residue(mp(ratfun)); double([xi , alpha]) double(alpha0)

ans = -9.999999999999628e-01 8.999999999998803e-01 9.999999999999993e-01 -1.666666666666714e-01 3.999999999999999e+00 2.666666666666091e-01 ans = -1.749809464778146e-17

The absolute term is close to zero because is of subdiagonal type. The output `cnd` of `residue` corresponds to the condition number of the transformation to partial fraction form. In this case of a low-order rational function with well separated poles the condition number is actually quite moderate:

cnd

cnd = 5.615057038451497e+01

## Conversion to quotient and continued fraction form

Our toolbox also implements the conversion of a `rkfun` to quotient form with two polynomials and given in the monomial basis. As with the conversion to partial fraction form, we recommend performing this transformation in multiple precision arithmetic due to potential ill-conditioning. Here we convert into the form and evaluate it at the root using MATLAB's `polyval` command:

[p,q] = poly(double(ratfun)); polyval(p, 2)./polyval(q, 2)

ans = -1.273055734903527e-14

A `rkfun` can also be converted into continued fraction form

$$ \displaystyle r(z) = h_0 + \cfrac{1}{\hat h_1+\cfrac{1}{h_1+\cfrac{1}{\hat h_2+\cdots+\cfrac{1}{h_{m-1}+\cfrac{1}{\hat h_m z+\cfrac{1}{h_m}}}}}} $$

as follows:

[h, hhat, absterm, cnd] = contfrac(mp(ratfun));

That's it for this tutorial. Note that more methods will be added over time and we'd be happy to receive any feedback or bug reports. For more details about the internal representation of `rkfun`, see [3].

The following command creates a thumbnail.

```
figure(3), hold on, plot(NaN)
```

## References

[1] Advanpix LLC., *Multiprecision Computing Toolbox for MATLAB,* ver 4.3.3.12213, Tokyo, Japan, 2017. http://www.advanpix.com/.

[2] M. Berljafa and S. Güttel. *Generalized rational Krylov decompositions with an application to rational approximation,* SIAM J. Matrix Anal. Appl., 36(2):894--916, 2015.

[3] M. Berljafa and S. Güttel. *The RKFIT algorithm for nonlinear rational approximation,* SIAM J. Sci. Comput., 39(5):A2049--A2071, 2017.

[4] T. A. Driscoll, N. Hale, and L. N. Trefethen. *Chebfun Guide,* Pafnuty Publications, Oxford, 2014. http://www.chebfun.org