  The N=20 and N=28 "islands of inversion" are described by large scale shell
model calculations with an extension of the interaction sdpf-u that makes it
possible to mix configurations with different Nhw or equivalently with
different number of particles promoted from the sd-shell to the pf-shell. It
allows to connect the classical sd-shell calculations below N=18 with the
sd(protons)-pf(neutrons) calculations beyond N=24-26, for all the isotopes from
Oxygen to Calcium, using the same interaction. For some isotopes this range
contains all the nuclei between the proton and the neutron drip lines and
includes the N=20 and N=28 islands of inversion. We shall pay particular
attention to the properties of the states at fix Nhw which turn out to be the
real protagonists of the physics at N=20. The existence of islands of
inversion/deformation will be explained as the result of the competition
between the spherical mean field which favors the 0hw configurations and the
nuclear correlations which favor the deformed Nhw configurations. The Magnesium
chain is exceptional because in it, the N=20 and N=28 "islands of inversion"
merge, enclosing all the isotopes between N=19 and N=30. Indeed, this would be
also the case for the Neon and Sodium chains if their drip lines would reach
N=28.
