“…Finally, among the many other works studying general qualitative properties of the problem (1.1)-(1.4), we wish to mention e.g. the papers [6], [7] (by Deuring, Kračmar and Nečasová), [10] (by Farwig), [13] (by Farwig, Krbec and Nečasová), [19], [20], (by Galdi), [23] (by Galdi and Silvestre), [26] (by Geissert, Heck and Hieber), [30], [32] (by Hishida) and [33] (by Hishida and Shibata).…”