The theory of differential operator equations has been described in various monographs. But the initial physical problem which leads to these equations is often hidden. When the physical problem is studied, the mathematical proofs are either not given or are quickly explained In this book, we give a systematic treatment of the differential equations with application to partial differential equations obtained from elastostatic problems. In particular, we study problems which are obtained from asymptotic expansion with two scales. We approximate and, when it is possible, expand the solution of problems by elementary solutions. This book is intended for scientists (mathematicians in the field of ordinary and partial differential equations, differential-operator equations; theoretical mechanics; theoretical physicists) and graduate students in Functional Analysis, Differential Equations, Equations of Mathematical Physics, and related topics.