Linear systems of ordinary differential equations with an identically singular or rectangular matrix multiplying the derivative of the desired vector function are considered. The structure of general solutions is discussed. A special case of perturbed systems is studied by applying the Vishik-Lyusternik method.

We study linear inhomogeneous vector ordinary differential equations of arbitrary order in which the matrix multiplying the highest derivative of the unknown vector function is singular in the domain

We consider systems of higher order integro-differential equations in which the matrix multiplying the highest derivative of the unknown vector function is identically singular in the domain where the system is considered. We give criteria for the solvability of such systems and initial value problems for these systems as well as examples illustrating the theoretical results.