Estimation of the pair correlation function and other quantities
by smoothing methods requires a choice of the smoothing bandwidth.
Stoyan and Stoyan (1995, equation (15.16), page 285) proposed a
rule of thumb for choosing the smoothing bandwidth.
For the Epanechnikov kernel, the rule of thumb is to set
the kernel's half-width \(h\) to
\(0.15/\sqrt{\lambda}\) where
\(\lambda\) is the estimated intensity of the point pattern,
typically computed as the number of points of X divided by the
area of the window containing X.
For a general kernel, the corresponding rule is to set the
standard deviation of the kernel to
\(\sigma = 0.15/\sqrt{5\lambda}\).
The coefficient \(0.15\) can be tweaked using the
argument co.