Thats a unique idea.You should consider the following idea to enhance your idea so that it would impact uniquely: The difference or a linear combination of two space-time 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  obtain flexible classes of spatial or spatio-temporal covariance functions with various properties such as long-range dependence and having different signs.
['Chunsheng Ma']
I am writing a research paper on a idea Linear combinations of space-time covariance functions and variograms
[{"from": "human", "value": "I am writing a research paper on a idea Linear combinations of space-time covariance functions and variograms"}, {"from": "gpt", "value": "Thats a unique idea.You should consider the following idea to enhance your idea so that it would impact uniquely: The difference or a linear combination of two space-time 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  obtain flexible classes of spatial or spatio-temporal covariance functions with various properties such as long-range dependence and having different signs."}]