“…Boundary value problems for elliptic differential-operator equations were considered in monographs by Krein [16], Dezin [10], Gorbachuk and Gorbachuk [15], Yakubov [23,24], Yakubov and Yakubov [27], Shklyar [20] and in many papers, for example, by Amann [5], deLaubenfels [8], Aibeche [1,2], Yakubov and Aliev [26], Dore and Yakubov [11], Yakubov [25], Aliev and Yakubov [4] and others. All these studies, usually, treat problems in Hilbert spaces.…”