  O(N) linear $\sigma$ model is solvable in the large $N$ limit and hence
provides a useful theoretical laboratory to test various unitarization
approximations. We find that the large $N_c$ limit and the $m_\sigma\to \infty$
limit do not commute. In order to get the correct large $N_c$ spectrum one has
to firstly take the large $N_c$ limit. We argue that the $f_0(600)$ meson may
not be described as generated dynamically. On the contrary, it is most
appropriately described at the same level as the pions, i.e, both appear
explicitly in the effective lagrangian. Actually it is very likely the $\sigma$
meson responsible for the spontaneous chiral symmetry breaking in a lagrangian
with linearly realized chiral symmetry.
