%% 8. Convergence for analytic functions
ATAPformats
%%
%
% Suppose $f$ is not just $k$ times differentiable but infinitely
% differentiable and in fact analytic on $[-1,1]$. (Recall that this means
% that for any $s\in [-1,1]$, $f$ has a Taylor series about $s$ that
% converges to $f$ in a neighborhood of $s$.) Then without any further
% assumptions we may conclude that the Chebyshev projections and
% interpolants converge {\bf geometrically}, that is, at the rate
% $O(C^{-n})$ for some constant $C>1$. This means the errors will look
% like straight lines (or better) on a semilog scale rather than a loglog
% scale. This kind of connection was first announced by Bernstein in 1911,
% who showed that the best approximations to a function $f$ on $[-1,1]$
% converge geometrically as $n\to\infty$ if and only if $f$ is analytic
% [Bernstein 1911 \& 1912{\sc b}].
%
%%
% For example, for Chebyshev interpolants of the function $(1+25x^2)^{-1}$,
% known as the _Runge function_ (Chapter 13), we get steady geometric
% convergence down to the level of rounding errors:
%%
% \vskip -2em
x = chebfun('x'); f = 1./(1+25*x.^2); nn = 0:10:200; ee = 0*nn;
for j = 1:length(nn)
n = nn(j); fn = chebfun(f,n+1); ee(j) = norm(f-fn,inf);
end
hold off, semilogy(nn,ee,'.'), grid on, axis([0 200 1e-17 10])
FS = 'fontsize';
title(['Geometric convergence of Chebyshev ' ...
' interpolants -- analytic function'],FS,9)
%%
% \vskip 1pt
%%
% If $f$ is analytic not just on $[-1,1]$ but in the whole complex
% plane---such a function is said to be _entire_---then the convergence is
% even faster than geometric. Here, for the function $\cos(20x)$, the dots
% are not approaching a fixed straight line but a curve that gets steeper
% as $n$ increases, until rounding errors cut off the progress.
%%
% \vskip -2em
f = cos(20*x); nn = 0:2:60; ee = 0*nn;
for j = 1:length(nn)
n = nn(j); fn = chebfun(f,n+1); ee(j) = norm(f-fn,inf);
end
semilogy(nn,ee,'.'), grid on, axis([0 60 1e-16 100])
title('Convergence of Chebyshev interpolants -- entire function',FS,9)
%%
% \vskip 1pt
%%
%
% There are elegant theorems that explain these effects. If $f$ is analytic
% on $[-1,1]$, then it can be analytically continued to a neighborhood of
% $[-1,1]$ in the complex plane. (The idea of analytic continuation is
% explained in complex variables textbooks; see also Chapter 28.) The
% bigger the neighborhood, the faster the convergence. In particular, for
% polynomial approximations, the neighborhoods that matter are the regions
% in the complex plane bounded by ellipses with foci at $-1$ and $1$,
% known as {\em Bernstein ellipses}
% [Bernstein 1912{\sc b}, 1912{\sc c} \&
% 1914{\sc a}]. It is easy to plot these curves: pick a number $\rho>1$
% and plot the image in the complex $x$-plane of the circle of radius
% $\rho$ in the $z$-plane under the Joukowsky map $x = (z+z^{-1})/2$. We
% let $E_\rho$ denote the open region bounded by this ellipse. Here, for
% example, are the Bernstein ellipses corresponding to the parameters $\rho
% = 1.1,1.2,\dots,2$:
%
%%
% \vskip -2em
z = exp(2i*pi*x);
for rho = 1.1:0.1:2
e = (rho*z+(rho*z).^(-1))/2; plot(e), hold on
end
ylim([-.9 .9]), axis equal
title('Bernstein ellipses for \rho = 1.1, 1.2, ..., 2',FS,9)
%%
% \vskip 1pt
%%
% It is not hard to verify that the length of the semimajor axis of
% $E_\rho$ plus the length of the semiminor axis is equal to $\rho$
% (Exercise 8.1).
%%
% Here is the basic bound on Chebyshev coefficients of analytic functions
% from which many other things follow. It first appeared in Section 61 of
% [Bernstein 1912b].
%%
% \em
% {\bf Theorem 8.1. Chebyshev coefficients of analytic functions.} Let a
% function $f$ analytic in $[-1,1]$ be analytically continuable to the open
% Bernstein ellipse $E_\rho$, where it satisfies $|f(x)|\le M$ for some
% $M$. Then its Chebyshev coefficients satisfy $|a_0| \le M$ and
% $$ | a_k | \le 2M\rho^{-k},\quad k\ge 1. \eqno (8.1) $$
% \vspace{0em}
%%
% _Proof._ As in the proofs of Theorems 3.1, 4.1, and 7.1, we make use of
% the transplantation from $f(x)$ and $T_k(x)$ on $[-1,1]$ in the $x$-plane
% to $F(z)$ and $(z^k + z^{-k})/2$ on the unit circle in the $z$-plane,
% with $x = (z+z^{-1})/2$ and $F(z) = F(z^{-1}) = f(x)$. The ellipse
% $E_\rho$ in the $x$-plane corresponds under this formula in a 1-to-2
% fashion to the annulus $\rho^{-1} < |z| < \rho$ in the $z$-plane. By
% this we mean that for each $x$ in $E_\rho\backslash [-1,1]$ there are two
% corresponding values of $z$ which are inverses of one another, and both
% the circles $|z|=\rho$ and $|z|=\rho^{-1}$ map onto the ellipse itself.
% (We can no longer use the formula $x = \hbox{Re}\,z$, which is valid only
% for $|z|=1$.) The first thing to note is that if $f$ is analytic in the
% ellipse, then $F$ is analytic in the annulus since it is the composition
% of the two analytic functions $z \mapsto (z+z^{-1})/2$ and $x \mapsto
% f(x)$. Now we make use of the contour integral formula (3.12),
% $$ a_k = {1\over\pi i}\int_{|z|=1}z^{-1-k}F(z)\,dz, $$
% with $\pi i$ replaced by $2\pi i$ for $k=0$. Suppose for a moment that
% $F$ is analytic not just in the annulus but in its closure $\rho^{-1} \le
% |z| \le \rho$. Then we can expand the contour to $|z|=\rho$ without
% changing the value of the integral, giving
% $$ a_k= {1\over\pi i}\int_{|z|=\rho}z^{-1-k}F(z)\,dz, $$
% again with $\pi i$ replaced by $2\pi i$ for $k=0$. Since the
% circumference is $2\pi \rho$ and $|F(z)| \le M$, the required bound now
% follows from an elementary estimate. If $F$ is analytic only in the open
% annulus, we can move the contour to $|z|=s$ for any $s<\rho$, leading to
% the same bound for any $s<\rho$ and hence also for $s=\rho$.
% $~\hbox{\vrule width 2.5pt depth 2.5 pt height 3.5 pt}$
%%
% Here are two of the consequences of Theorem 8.1. Equation (8.2) first
% appeared in [Bernstein 1912b, Sec. 61]. I do not know where equation
% (8.3) may have appeared, though similar slightly weaker bounds can be
% found in (4.13) and (4.16) of [Tadmor 1986]. For a generalization of
% (8.3) to interpolation in other point sets with the same asymptotic
% distribution as Chebyshev points, see Theorem 12.1.
%%
% \em
% {\bf Theorem 8.2. Convergence for analytic functions.}
% If $f$ has the properties of Theorem $8.1$, then for
% each $n\ge 0$ its Chebyshev projections satisfy
% $$ \| f - f_n \| \le {2M\rho^{-n}\over \rho - 1} \eqno (8.2) $$
% and its Chebyshev interpolants satisfy
% $$ \| f - p_n \| \le {4M\rho^{-n}\over \rho - 1}. \eqno (8.3) $$
% \vspace{-1.5em}
%%
% _Proof._ Equation (8.2) follows from Theorem 8.1 and (4.8),
% and (8.3) follows from Theorem 8.1 and (4.9).
% $~\hbox{\vrule width 2.5pt depth 2.5 pt height 3.5 pt}$
%%
% We can apply Theorem 8.2 directly if $f$ is analytic and bounded in
% $E_\rho$. If it is analytic but unbounded in $E_\rho$, then it will be
% analytic and bounded in $E_s$ for any $s<\rho$, so we still get
% convergence at the rate $O(s^{-n})$ for any $s<\rho$. If it is unbounded
% in $E_\rho$ but the only singularities on the ellipse are simple poles,
% then we get convergence at the rate $O(\kern .5pt\rho^{-n})$ after all
% (Exercise 8.15).
%%
%
% Before applying Theorem 8.2 to a couple of examples, it will be
% convenient to note formulas for $\rho$ in two common special cases.
% Suppose $f$ has its first singularity at a real value $x_0 = \pm \alpha$
% for some $\alpha>1$. Then the corresponding ellipse parameter is
% $$ \rho = \alpha + \sqrt{\alpha^2-1} \quad \hbox{(real singularity at $x = \pm \alpha$).}
% \eqno (8.4) $$
% Or suppose that the first singularity is at the pure imaginary value
% $x_0 = \pm i\beta$ for some $\beta>0$. Then we have
% $$ \rho = \beta + \sqrt{\beta^2+1} \quad \hbox{(imaginary singularity at $x = \pm i\beta$).}
% \eqno (8.5) $$
%
%%
% For example, the Runge function $(1+25x^2)^{-1}$ considered above has
% poles at $\pm i/5$. By (8.5), the corresponding value of $\rho$ is
% $(1+\sqrt{26}\kern .7pt )/5 \approx 1.220$, and the errors in Chebyshev
% interpolation match this rate beautifully:
%%
% \vskip -2em
f = 1./(1+25*x.^2); nn = 0:10:200; ee = 0*nn;
for j = 1:length(nn)
n = nn(j); fn = chebfun(f,n+1); ee(j) = norm(f-fn,inf);
end
rho = (1+sqrt(26))/5;
hold off, semilogy(nn,rho.^(-nn),'-r')
hold on, semilogy(nn,ee,'.'), grid on, axis([0 200 1e-17 10])
title('Geometric convergence for the Runge function',FS,9)
%%
% \vskip 1pt
%%
% Here is a more extreme but entirely analogous example: $\tanh(50 \pi x)$,
% with poles at $\pm 0.01i$. These poles are so close to $[-1,1]$ that the
% convergence is much slower, but it is still robust. The only difference
% in this code segment is that |norm(f-fn,inf)|, a relatively slow Chebfun
% operation that depends on finding zeros of the derivative of |f-fn|, has
% been replaced by the default 2-norm |norm(f-fn)|, which is quick. This
% makes little difference to the figure, as the exponential decay rates are
% the same. (In the $\infty$-norm, the dots in the figure would appear
% just above the red line instead of just below it.)
%%
% \vskip -2em
f = tanh(50*pi*x); nn = 0:200:4000; ee = 0*nn;
for j = 1:length(nn)
n = nn(j); fn = chebfun(f,n+1); ee(j) = norm(f-fn);
end
rho = (1+sqrt(10001))/100;
hold off, semilogy(nn,rho.^(-nn),'-r')
hold on, semilogy(nn,ee,'.')
grid on, axis([0 4000 1e-16 10])
title(['Geometric convergence for a function ' ...
'analytic in a narrow region'],FS,9)
%%
% \vskip 1pt
%%
% For an example with a real singularity, the function $\sqrt{2-x}$ has a
% branch point at $x=2$, corresponding by (8.4) to $\rho = 2+\sqrt{3}$.
% Again we see a good match, with the curve gradually bending over to the
% expected slope as $n\to\infty$.
%%
% \vskip -2em
f = sqrt(2-x);
nn = 0:30; ee = 0*nn;
for j = 1:length(nn)
n = nn(j); fn = chebfun(f,n+1); ee(j) = norm(f-fn,inf);
end
rho = 2+sqrt(3);
hold off, semilogy(nn,rho.^(-nn),'-r')
hold on, semilogy(nn,ee,'.')
grid on, axis([0 30 1e-17 10])
title(['Geometric convergence for an analytic ' ...
'function with a branch point'],FS,9)
%%
% \vskip 1pt
%%
% We now derive an elegant converse of Theorem 8.2, also due to Bernstein
% [1912b, Section 9]. The converse is not quite exact: Theorem 8.2 assumes
% analyticity and boundedness in $E_\rho$, whereas the conclusion of
% Theorem 8.3 is analyticity but not necessarily boundedness (Exercise 8.15).
%%
% *Theorem 8.3. Converse of Theorem 8.2.* _Suppose $f$ is a function on
% $[-1,1]$ for which there exist polynomial approximations $\{q_n\}$
% satisfying_
% $$ \| f - q_n \| \le C\rho^{-n}, \quad n\ge 0 $$
% _for some constants $\rho>1$ and $C>0$. Then $f$ can be analytically
% continued to an analytic function in the open Bernstein ellipse_
% $E_{\rho}$.
%%
% _Proof._ The assumption implies that the polynomials $\{q_n\}$ satisfy
% $\|q_n-q_{n-1}\|\le 2\kern .7pt C\kern .4pt \rho^{1-n}$ on $[-1,1]$.
% Since $q_n-q_{n-1}\in {\cal P}_n$, it can be shown that this implies
% $\|q_n - q_{n-1}\|_{E_s} \le 2\, C s^n \rho^{1-n} $ for any $s>1$, where
% $\|\cdot\|_{E_s}$ is the supremum norm on the $s$-ellipse $E_s$. (This
% estimate is one of _Bernstein's inequalities,_ from Section 9 of
% [Bernstein 1912b]; see Exercise 8.6.) For $s<\rho$, this gives us a
% representation for $f$ in $E_s$ as a series of analytic functions, $$ f =
% q_0 + (q_1-q_0) + (q_2-q_1) + \cdots , $$ which is uniformly convergent
% according to the Weierstrass M-test. According to another well-known
% theorem of Weierstrass, this implies that the limit is a bounded analytic
% function [Ahlfors 1953, Markushevich 1985]. Since this is true for any
% $s<\rho$, the analyticity applies throughout $E_\rho$.
% $~\hbox{\vrule width 2.5pt depth 2.5 pt height 3.5 pt}$
%%
% Note that Theorem 8.2 and 8.3 together establish a simple fact, sometimes
% known as _Bernstein's theorem_: a function defined on $[-1,1]$ can be
% approximated by polynomials with geometric accuracy if and only if it is
% analytic. (See also Exercise 8.11 and [Bagby & Levenberg 1993].)
%%
% The term ``Bernstein ellipse'' refers to any ellipse in the complex plane
% with foci $\{-1,1\}$, and if $f$ is a function analytic on $[-1,1]$, the
% bounds of Theorems 8.1 and 8.2 apply for any Bernstein ellipse inside
% which $f$ is analytic and bounded. If there is a largest ellipse inside
% which $f$ is analytic, then one might choose to say that this was ``the''
% Bernstein ellipse for $f$, but this might not always be the ellipse that
% gives the most useful bound, and if $f$ is entire, then there is no
% largest ellipse at all (Exercise 8.3).
%%
% Chebfun computations, however, suggest a practical way to single out a
% special Bernstein ellipse associated with a given function $f$. The
% _Chebfun ellipse_ for $f$ is the Bernstein ellipse whose parameter $\rho$
% satisfies the condition
% $$ \rho^{-n} = \varepsilon, \eqno (8.6) $$
% where
% $\varepsilon$ is the tolerance used by the Chebfun constructor (normally
% $2^{-52}$) and $n$ is the degree of the polynomial chosen by Chebfun to
% resolve $f$. The command |chebellipseplot| plots these Chebfun
% ellipses. Thus for $f(x) = 1/(1+25x^2)$, for example, the Chebfun ellipse
% comes very close to passing through the poles at $\pm 0.2i$:
%%
% \vskip -2em
f = chebfun('1./(1+25*x.^2)');
clf, chebellipseplot(f,'linewidth',1)
hold on, plot([.2i -.2i],'xr','markersize',12)
axis equal, ylim(.5*[-1 1]), grid on
title('Chebfun ellipse for 1/(1+25x^2)',FS,9)
%%
% \vskip 1pt
%%
% For the entire function $f(x) = \exp(-200x^2)$, the Chebfun ellipse has
% much the same shape although now $f$ has no singularities:
%%
% \vskip -2em
f = chebfun('exp(-200*x.^2)');
hold off, chebellipseplot(f,'linewidth',1)
axis equal, ylim(.5*[-1 1]), grid on
title('Chebfun ellipse for exp(-200x^2)',FS,9)
%%
% \vskip 1pt
%%
%
% \begin{displaymath}
% \framebox[4.7in][c]{\parbox{4.5in}{\vspace{2pt}\sl
% {\sc Summary of Chapter 8.} If $f$ is analytic, its Chebyshev
% coefficients $\{a_k\}$ decrease geometrically. In particular, if $f$ is
% analytic with $|f(x)| \le M$ in the Bernstein $\rho$-ellipse about
% $[-1,1]$, then $|a_k|\le 2M\rho^{-k}$. It follows that the degree $n$
% Chebyshev projection and interpolant of $f$ have accuracy
% $O(M\rho^{-n})$.\vspace{2pt}}}
% \end{displaymath}
%
%%
% \smallskip\small\parskip=2pt
% {\bf Exercise 8.1. Bernstein ellipses.} Verify that for any $\rho >1$,
% the length of the
% semiminor axis plus the length of the semimajor axis of the Bernstein
% ellipse $E_\rho$ is equal to $\rho$.
% \par
% {\bf Exercise 8.2. A Chebyshev series.}
% With {\tt x = chebfun('x')}, execute the command
% {\tt chebpolyplot(sin(100*(x-.1))+.01*tanh(20*x))}. Explain the various
% features of the resulting plot as quantitatively as you can.
% \par
% {\bf Exercise 8.3. Interpolation of an entire function.} The
% function $f(x) = \exp(-x^2)$ is analytic throughout the complex $x$-plane,
% so Theorem 8.2 can be applied for any
% value of the parameter $\rho>1$.
% Produce a semilog plot of $\|f-p_n\|$ as a function of $n$
% together with lines corresponding to the upper bound of the theorem for
% $\rho = 1.1, 1.2, 1.4, 2, 3, 5, 8$. Be sure to use the
% right value of $M$ in each case. How well do your bounds fit the data?
% \par
% {\bf Exercise 8.4. Convergence rates for different functions.}
% Based on the theorems of this chapter, what can you say about
% the convergence as $n\to\infty$ of the Chebyshev interpolants to
% (a) $\tan(x)$,
% (b) $\tanh(x)$,
% (c) $\log((x+3)/4)/(x-1)$,
% (d) $\int_{-1}^x \cos(t^2)\kern 1pt dt$,
% (e) $\tan(\tan(x))$,
% (f) $(1+x) \log(1+x)\,$?
% In each case compare theoretical bounds with numerically computed
% results. Which is the case that converges much faster than the
% theorems predict? Can you speculate as to why?
% \par
% {\bf Exercise 8.5. Accuracy of approximations in the complex plane.} Let
% $p$ be the chebfun for $f(x) = \exp(-200x^2)$ and plot contour lines in
% the complex $x$-plane corresponding to $|f(x) - p(x)| = 10^{-2}, 10^{-4},
% \dots, 10^{-14}$. How do these curves compare to the Bernstein ellipses
% corresponding to parameters $\rho$ satisfying $\rho^{-n} = \varepsilon
% \times \{ 10^2, 10^4, \dots , 10^{14}\}$, where $\varepsilon$ is the
% Chebfun constructor tolerance $2^{-52}\kern 1pt$?
% \par
% {\bf Exercise 8.6. Proof of Bernstein inequality.} Prove Bernstein's
% inequality used in the proof of Theorem 8.3: if $p$ is a polynomial of
% degree $d$, then $\|\kern .7pt p\|_{E_\rho} \le \rho^d \kern 1pt \|\kern
% .7pt p\|$, where $\|\cdot \|_{E_\rho}$ is the $\infty$-norm over the
% $\rho$-ellipse and $\|\cdot\|$ is the $\infty$-norm over $[-1,1]$. (Hint:
% Show that if the branch cut is taken to be the unit interval $[-1,1]$,
% the function $q(z) = p(z)/(z+(z^2-1)^{1/2})^d$ is analytic throughout the
% region consisting of the complex plane plus the point $z=\infty$ minus
% $[-1,1]$. Apply the maximum modulus principle.)
% \par
% {\bf Exercise 8.7. Absolute value function.}
% The function $|x-i|$ is analytic for $x\in [-1,1]$.
% This means it can be analytically continued to an analytic
% function $f(x)$ in a neighborhood of $[-1,1]$ in the complex $x$-plane.
% The formula $|x-i|$ itself does not define an analytic function
% in any complex neighborhood. Find another formula for $f$ that does,
% and use it to explain what singularities $f$ has in the complex plane.
% \par
% {\bf Exercise 8.8. Chebyshev polynomials on the Bernstein ellipse.}
% Show that for any $\rho>1$ and any $z$ on the boundary of the
% ellipse $E_\rho$ in the complex $x$-plane,
% $\lim_{n\to\infty} |T_n(x)|^{1/n} = \rho$.
% \par
% {\bf Exercise 8.9. You can't judge smoothness by eye.}
% Define $f(x) = 2+\sin(50x)$ and $g(x) = f(x)^{1.0001}$ and
% construct chebfuns for these functions on $[-1,1]$. What are
% their lengths? Explain this effect quantitatively
% using the theorems of this chapter.
% \par
% {\bf Exercise 8.10. Convergence of conjugate gradient iteration.}
% Suppose we wish to approximate $f(x) = x^{-1}$ on the interval $[m,M]$
% with $0 < m1$ with the only singularities
% on the ellipse itself being simple poles. Show that
% $\|f-f_n\|$ and $\|f-p_n\|$ are of size $O(\kern .5pt\rho^{-n})$ as $n\to\infty$.
% \par