Abstract
The aim of this short article is to stress the importance of using only positive-definite functions as models for covariance functions and variograms.
The two examples presented show that a negative variance can easily be obtained when a nonadmissible function is chosen for the variogram model.
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Armstrong, M., Jabin, R. Variogram models must be positive-definite. Mathematical Geology 13, 455–459 (1981). https://doi.org/10.1007/BF01079648
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DOI: https://doi.org/10.1007/BF01079648