“…There is an intensive literature on ASE (see e.g., [6,11,19,30,34,37] and many others). The literature on ASD is much less than ASE but has been growing quickly for the past 10 years (see e.g., [24,46,51,52]), in particular, several recent papers [21,22,50]. The reason why there are fewer papers on ASD than ASE is because the mathematics involved is much more complicated than that used for the study of ASE but certainly not because ASD is less important.…”