The Characterization Problem for Isotropic Covariance Functions |
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Authors: | Tilmann Gneiting and Zoltán Sasvári |
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Institution: | (1) Department of Statistics, University of Washington, Box 354322, Seattle, Washington, 98195-4322;(2) Department of Mathematics, Technische Universität Dresden, Mommsenstr. 13, D-01062 Dresden, Germany |
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Abstract: | Isotropic covariance functions are successfully used to model spatial continuity in a multitude of scientific disciplines. Nevertheless, a satisfactory characterization of the class of permissible isotropic covariance models has been missing. The intention of this note is to review, complete, and extend the existing literature on the problem. As it turns out, a famous conjecture of Schoenberg (1938) holds true: any measurable, isotropic covariance function on
d
(d 2) admits a decomposition as the sum of a pure nugget effect and a continuous covariance function. Moreover, any measurable, isotropic covariance function defined on a ball in d can be extended to an isotropic covariance function defined on the entire space
d
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Keywords: | correlation function extension theorem nugget effect positive definite radial Schoenberg's conjecture |
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