The difference or a linear combination of two spacetime covariance functions (or variograms) is not necessarily a valid covariance function (or variogram). In general, there seems no simple condition that makes a linear combination permissible, unless its coefficients are non-negative. The permissibility is investigated in this paper for the linear combination of two spatial or spatio-temporal covariance functions (or variograms) isotropic in space so that we obtain flexible classes of spatial or spatio-temporal covariance functions with various properties such as long-range dependence and having different signs.
机构:
Khalifa Univ, Dept Math, Abu Dhabi, U Arab EmiratesKhalifa Univ, Dept Math, Abu Dhabi, U Arab Emirates
Porcu, Emilio
White, Philip A.
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Berry Consultants, Austin, TX USA
Brigham Young Univ, Dept Stat, Provo, UT USAKhalifa Univ, Dept Math, Abu Dhabi, U Arab Emirates
White, Philip A.
Genton, Marc G.
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King Abdullah Univ Sci & Technol, Stat Program, Thuwal, Saudi Arabia
King Abdullah Univ Sci & Technol, Stat Program, Thuwal 239556900, Saudi ArabiaKhalifa Univ, Dept Math, Abu Dhabi, U Arab Emirates