  We prove that in the limit of the coupling going to infinity a Yang-Mills
theory is equivalent to a $\lambda\phi^4$ theory with the dynamics ruled just
by a homogeneous equation. This gives explicitly the Green function and the
mass spectrum proving that such gauge theories are confining. The scalar
glueball spectrum is then proven to be in fair agreement with lattice QCD
computations but giving a different ground state coinciding with the $f_0(600)$
light unflavored meson.
