  Qualitative and spectral properties of the form-sums
  S_{\pm}(V):=D_{\pm}^{2m}\dotplus V(x),\quad m\in \mathbb{N}, in the Hilbert
space $L_{2}(0,1)$ are studied. Here the periodic $(D_{+})$ and the
semiperiodic $(D_{-})$ differential operators are $D_{\pm}: u\mapsto -i u'$,
and $V(x)$ is a 1-periodic complex-valued distribution in the Sobolev spaces
$H_{per}^{-m\alpha}$, $\alpha\in [0,1]$.
