  We report the discovery of a set of four-point, two-factor, free-ranging,
putatively IMSPE-optimal designs with a pair of twin points, in the statistical
design of computer experiments, under Gaussian-process,
fixed-Gaussian-covariance parameter, and zero-nugget assumptions. We conjecture
this is the set of free-ranging, twin-point designs with the smallest number of
degrees of freedom.

Methodology