  We establish a Floquet theorem for a first-order system of differential
equations $u'=ru$ where $r$ is an $n\times n$-matrix whose entries are periodic
distributions of order $0$. Then we investigate, when $n=1$ and $n=2$, the
spectral theory for the equation $Ju'+qu=wf$ on $\mathbb R$ when $J$ is a real,
constant, invertible, skew-symmetric matrix and $q$ and $w$ are periodic
matrices whose entries are real distributions of order $0$ with $q$ symmetric
and $w$ non-negative.
