Bohr and Mottelson  pointed out that, under the adiabatic approximation, the rotational energy of an axially symmetric even-even nucleus
may be expanded as (for k = 0, where k is the projection of the angular momentum I onto the symmetric axis) a power series in terms of [I.
In the view of the above mentioned, it seems that the ground state energy bands of deformed even-even nucleus
have quantum number K=0 (K is the projection of I along the symmetry axis), together with even parity and angular momentum.
We note that in the interacting boson model-1 (IBM-1) [1,2] one describes an even-even nucleus
as a system of N bosons able to occupy two levels, one with angular momentum restricted to zero (s boson) and one with angular momentum 2 (d boson).