Halphen's constant for approximation of exp(x)

Nick Trefethen, May 2011

(Chebfun example approx/Halphen.m)

A well-known problem in approximation theory is, how well can exp(x) be approximated in the infinity norm on the infinite interval (-infty,0] by rational functions of type (n,n)? To three places, the first few approximation errors are these:

Type (0,0):  error = 0.500
Type (1,1):  error = 0.0668
Type (2,2):  error = 0.00736
Type (3,3):  error = 0.000799
Type (4,4):  error = 0.0000865
Type (5,5):  error = 0.00000934
Type (6,6):  error = 0.000001008
Type (7,7):  error = 0.0000001087
Type (8,8):  error = 0.00000001172

As n increases to infinity, it is known that the asymptotic behavior is

  error ~ 2 C^(-n-1/2),

where C is a number known as Halphen's constant with the following approximate numerical value:

halphen_const = 9.289025491920818918755449435951
halphen_const =

This result comes from a sequence of contributions between 1969 and 2002 by, among others, Cody, Meinardus and Varga; Newman; Trefethen and Gutknecht; Carpenter, Ruttan and Varga; Magnus; Gonchar and Rakhmanov; and Aptekarev.

Here is a plot showing that the asymptotic behavior matches the actual errors very closely even for small n:

LW = 'linewidth'; MS = 'markersize'; FS = 'fontsize';
n = 0:10;
err = [.5 .0668 7.36e-3 7.99e-4 8.65e-5 9.35e-6 ...
       1.01e-6 1.09e-7 1.17e-8 1.26e-9 1.36e-10];
model = 2*halphen_const.^(-n-.5);
hold off, semilogy(n,model,'-b',LW,1.2)
hold on, semilogy(n,err,'.k',MS,18), grid on
xlabel('n',FS,14), ylabel('error',FS,14)

One way to characterize Halphen's constant mathematically is that it is the inverse of the unique positive value of s where the function

  SUM from k=1 to infty of  ks^n/(1-(-s)^n)

takes the value 1/8. This is an easy computation for Chebfun:

s = chebfun('s',[1/12,1/6]);
f = 0*s; k = 0; normsk = 999;
while normsk > 1e-16
  k = k+1;
  sk = s.^k;
  f = f + k*sk./(1-(-1)^k*sk);
  normsk = norm(sk,inf);
hold off, plot(1./s,f,LW,1.2), grid on
h = 1/roots(f-1/8);
hold on, plot(h,1/8,'.r',MS,20)
title('Halphen''s constant',FS,14)


[1] A. J. Carpenter, A. Ruttan, and R. S. Varga, Extended numerical computations on the "1/9" conjecture in rational approximation theory, in P. Graves-Morris, E. B. Saff, and R. S. Varga, eds., Rational Aprpoximation and Interpolation, Lecture Notes in Mathematics 1005, Springer, 1984.

[2] A. A. Gonchar and E. A. Rakhmanov, Equilibrium distributions and degree of rational approximation of analytic functions, Math. USSR Sbornik 62 (1989), 305-348.

[3] L. N. Trefethen, Approximation Theory and Approximation Practice, draft available at http://www.maths.ox.ac.uk/chebfun/ATAP (chapter 24).