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This textbook is an introduction to the subject of ordinary differential equations (ODEs). This book is intended for the student at the sophomore-level who has successfully completed the first two semesters of a typical college or university calculus sequence and who intends to enroll in engineering or mathematics courses or any one of the physical sciences courses involving ODEs---but it will also serve the student who will be taking courses in other disciplines that occasionally employ ODEs, such as actuarial science, econometrics, computational science, and operational research