Exponential node clustering at singularities for rational approximation,
quadrature, and PDEs

Rational approximations of functions with singularities can converge at a
root-exponential rate if the poles are exponentially clustered. We begin by
reviewing this effect in minimax, least-squares, and AAA approximations on
intervals and complex domains, conformal mapping, and the numerical solution of
Laplace, Helmholtz, and biharmonic equations by the "lightning" method.
Extensive and wide-ranging numerical experiments are involved. We then present
further experiments showing that in all of these applications, it is
advantageous to use exponential clustering whose density on a logarithmic scale
is not uniform but tapers off linearly to zero near the singularity. We give a
theoretical explanation of the tapering effect based on the Hermite contour
integral and potential theory, showing that tapering doubles the rate of
convergence. Finally we show that related mathematics applies to the
relationship between exponential (not tapered) and doubly exponential (tapered)
quadrature formulas. Here it is the Gauss--Takahasi--Mori contour integral that
comes into play.