“…In recent years, highly important results in this field have been obtained for the case when the eigenparameter appears not only in the differential equation but also in the boundary conditions. The literature on such results is voluminous and we refer to [1,2,4,5,7,8,11,13,14]. While the general theory and methods of such second-order boundary value problems are highly developed, very little is known about the general character of the higher order problems.…”