CONSTRUCTION OF FUNCTION-SPACES BY MEANS OF THE LAPLACIAN AND THE AVERAGING METHOD Статья ZHIDKOV G.V. Differential Equations. Том 20. 1984. С. 477-485
VARIATIONAL METHOD FOR SOLVING BOUNDARY-VALUE-PROBLEMS FOR THE WAVE-EQUATION Статья FILIPPOV V.M. Differential Equations. Том 20. 1984. С. 1401-1407