  A microscopic explanation of the nature of shape coexistence in the N=90,
Z=64 region is suggested, based on calculations of single particle energies
through standard covariant density functional theory. It is suggested that
shape coexistence in the N=90 region is caused by the protons, which create
neutron particle-hole (p-h) excitations across the N=112 3-dimensional
isotropic harmonic oscillator (3D-HO) magic number, signaling the start of the
occupation of the 1i13/2 intruder orbital, which triggers stronger
proton-neutron interaction, causing the onset of the deformation and resulting
in the shape/phase transition from spherical to deformed nuclei described by
the X(5) critical point symmetry. A similar effect is seen in the N=60, Z=40
region, in which p-h excitations across the N=70 3D-HO magic number occur,
signaling the start of the occupation of the 1h11/2 intruder orbital.
