“…While various kernels have been proposed (e.g., [5,6,7]), three most widely used kernels are the so-called Stable Spline kernel (SS) [2], the Tuned-Correlated kernel (sometimes also called the firstorder stable spline kernel) [8], and the Diagonal-Correlated (DC) kernel [8]. These three kernels have simple structures and favorable properties, and their effectiveness are shown in various works, e.g., [8,9,10,11].…”