On the Defining Boundary Conditions of Elliptic Boundary Value Problems with a Small Parameter at the Highest Derivatives

The concepts of associated and defining boundary conditions are introduced for elliptic boundary value problems with a small parameter at the highest derivatives. An explicit expression is obtained for the associated boundary conditions in the case when the difference between the orders of the perturbed and degenerate equations is equal to 4. Theorems on asymptotic expansion are proved.