This paper studies the eigenfunction expansion method to solve the twodimensional (2D) elasticity problems based on the stress formulation. The fundamental system of partial differential equations of the 2D problems is rewritten as an upper triangular differential system based on the known results, and then the associated upper triangular operator matrix matrix is obtained. By further research, the two simpler complete orthogonal systems of eigenfunctions in some space are obtained, which belong to the two block operators arising in the operator matrix. Then, a more simple and convenient general solution to the 2D problem is given by the eigenfunction expansion method. Furthermore, the boundary conditions for the 2D problem, which can be solved by this method, are indicated. Finally, the validity of the obtained results is verified by a specific example.
224EBURILITU and ALATANCANG method of separation of variables based on the Hamiltonian system. Moreover, given a mechanics equation, in general, we can look for the various differential systems [12] . The Hamiltonian system is only one class of them. In all differential systems, we believe that the upper triangular differential systems have certain advantages in solving which problems. Now, many researchers are studying the upper triangular operator matrices, which has become a research hot point of the operator theory.In Ref.[13], the fundamental system of partial differential equations of the 2D problems is rewritten as an upper triangular differential system. For the associated operator matrix, the existence of the two the complete orthogonal systems of eigenfunctions in some space are obtained, which belong to the two block operators arising in the operator matrix. The general solution to the 2D problem is given by the eigenfunction expansion method. However, the analytic expression formula of the general solution is very tedious and complicate. Therefore, it is inconvenient to be applied.Through an in-depth study of the 2D problem, in a new coordinate system, this paper presents a more simple and practical general solution. Furthermore, the boundary conditions for the 2D problem, which can be solved by the method, are indicated.