Elementary Differential Equations & Operators By Reuter. The main purpose of this book is to give a thoroughly elementary account of the ‘operational method’ for solving linear differential equations with constant coefficients, subject to prescribed initial conditions. No previous knowledge of differential equations will be demanded from the reader, and accordingly the basic properties of linear differential equations with constant coefficients are treated in the first chapter. This chapter may be found useful even by those readers who do not wish to leam the more specialized technique of operators, treated in the second chapter.