  We study functional and spectral properties of perturbations of the magnetic
Laplace operator on the circle. This operator appears when considering the
restriction to the unit circle of a two-dimensional Schr{\"o}dinger operator
with the Bohm-Aharonov vector potential. We prove a Hardy-type inequality on
the two-dimensional Euclidean space and, on the circle, a sharp interpolation
inequality and a sharp Keller-Lieb-Thirring inequality.
