Especially, the case n = 1 in (I) has been enthusiastically considered in many studies. For this we only refer to the papers of Lu [Lu], Cao and Dong [CaDo], the first author [Mi2], the second author [Luc], and the references given there
Especially,the case n = 1 in (I) has been enthusiastically considered in many studies. For this weonly refer to the papers of Lu [Lu], Cao and Dong [CaDo], the first author [Mi2], thesecond author [Luc], and the references given there