Contours and dimple for the Gneiting class of space-time correlation functions
研究了Gneiting类时空相关函数中凹陷性质与等高线的对应关系，发现凹陷与参数曲线的非单调性一一对应，并提出了无凹陷的修正Gneiting类。
We offer a dual view of the dimple problem related to space-time correlation functions in terms of their contours. We find that the dimple property (Kent et al., 2011) in the Gneiting class of correlations is in one-to-one correspondence with nonmonotonicity of the parametric curve describing the associated contour lines. Further, we show that given such a nonmonotonic parametric curve associated with a given level set, all the other parametric curves at smaller levels inherit the nonmonotonicity. We propose a modified Gneiting class of correlations having monotonically decreasing parametric curves and no dimple along the temporal axis.