A large amount of work has been done on ordinary differ- ential equations with small parameters multiplying deriv- atives. This book investigates questions related to the asymptotic calculation of relaxation oscillations, which are periodic solutions formed of sections of both sl- and fast-motion parts of phase trajectories. A detailed discussion of solutions of differential equations involving small parameters is given for regions near singular points. The main results examined were obtained by L. S. Pontryagin and the authors. Other works have also been taken into account: A. A....