Green's functions for first-order systems of ordinary differential equations without the unique continuation property

This paper is a contribution to the spectral theory associated with the differential equation $J{u}^{\mathrm{\prime }}+qu=wf$ on the real interval $(a,b)$ when $J$ is a constant, invertible skew-Hermitian matrix and $q$ and $w$ are matrices whose entries are distributions of order zero with $q$ Hermitian and $w$ non-negative. Under these hypotheses it may not be possible to uniquely continue a solution from one point to another, thus blunting the standard tools of spectral theory. Despite this fact we are able to describe symmetric restrictions of the maximal relation associated with $J{u}^{\mathrm{\prime }}+qu=wf$ and show the existence of Green’s functions for self-adjoint relations even if unique continuation of solutions fails.