R. Conti & G. Sansone 
Non-Linear Differential Equations [PDF ebook] 

Non-Linear Differential Equations covers the general theorems, principles, solutions, and applications of non-linear differential equations. This book is divided into nine chapters. The first chapters contain detailed analysis of the phase portrait of two-dimensional autonomous systems. The succeeding chapters deal with the qualitative methods for the discovery of periodic solutions in periodic systems. The remaining chapters describe a synthetical approach to the study of asymptotic properties, especially stability properties, of the solutions of general n-dimensional systems. This book will be of great value to mathematicians, researchers, and students.