  Since the order of elliptic type model equation (Laplace equation) is two
[1], [2], then it is natural the order of composite type model equation must be
[3] [4] [5] three. At each point of the domain under consideration these
equations have both real and complex characteristics. Notice that a boundary
value problem for a composite type equation of second order first appeared in
the paper [6]. The method for investigating the Fredholm property of boundary
value problems is distinctive and belongs to one of the authors of the present
paper.
