Both kernels in Equations 3 are isotropic, of the form K d,a ð Þ~1=(2pd)|f d; a ð Þ, where f d; a ð Þ is a one-dimensional kernel defined on the positive real axis (for the kernels in Equations 3, f d; a ð Þ is a negative exponential and half-Cauchy kernel, respectively).