covariance structure rational quadratic anisotropic covariance structure Alejandra Gonzalez-Beltran Orlaith Burke Camille Maumet Philippe Rocca-Serra The rational quadratic covariance function is used in spatial statistics, geostatistics, machine learning, image analysis, and other fields where multivariate statistical analysis is conducted on metric spaces. It is commonly used to define the statistical covariance between measurements made at two points that are d units distant from each other. Since the covariance only depends on distances between points, it is stationary. If the distance is Euclidean distance, the rational quadratic covariance function is also isotropic. Tom Nichols http://stat.ethz.ch/R-manual/R-devel/library/nlme/html/corRatio.html http://support.sas.com/documentation/cdl/en/statug/63033/HTML/default/viewer.htm#statug_mixed_sect019.htm https://en.wikipedia.org/wiki/Rational_quadratic_covariance_function metadata complete