  We study axisymmetric perturbations of neutron star endowed with a strong
magnetic field (magnetars), considering the coupled oscillations of the fluid
core with the solid crust. We recover discrete oscillations based mainly in the
crust and a continuum in the core. We also confirm the presence of "discrete
Alfv\'en modes" in the gap between two contiguous continua (see Van Hooven&
Levin (2011)) and, in addition, we can resolve some of them also inside the
continua. Our results can explain both the lower and the higher observed quasi
periodical oscillations (QPOs) in SGR 1806-20 and SGR 1900+14 and put
constrains on the mass, radius and crust thickness of the two magnetars.
