“…where σ 2 = ψ −1 2 . Here it is important to remind the reader that in this paper our motive of introducing the kernels k 1 and k 2 is entirely different from the other existing works involving circular and spherical data, where the goals are density estimation, nonparametric regression and smoothing (see, for example, Hall et al (1987), , ), Marzio et al (2011), Marzio et al (2012b, Marzio et al (2012a), Marzio et al (2014)). Hence, our kernels need not satisfy the optimality properties required for the aforementioned works.…”