The development of the theory of ordinary differential equations with a small parameter multiplying the highest derivative during the period 1966–1976

We survey the results relating to the theory of singular perturbations, in particular, to the theory of ordinary differential equations with a small parameter multiplying the highest derivative, that have been obtained by pupils of A. N. Tikhonov during the period 1966-1976. The main attention is directed at problems in which the roots of the characteristic equation have real parts of different signs (the conditionally stable case), and also at problems in which the characteristic equation has roots that are identically zero.