  Low-energy limit of quantum chromodynamics (QCD) is obtained using a mapping
theorem recently proved. This theorem states that, classically, solutions of a
massless quartic scalar field theory are approximate solutions of Yang-Mills
equations in the limit of the gauge coupling going to infinity. Low-energy QCD
is described by a Yukawa theory further reducible to a Nambu-Jona-Lasinio
model. At the leading order one can compute glue-quark interactions and one is
able to calculate the properties of the $\sigma$ and $\eta-\eta'$ mesons.
Finally, it is seen that all the physics of strong interactions, both in the
infrared and ultraviolet limit, is described by a single constant $\Lambda$
arising in the ultraviolet by dimensional transmutation and in the infrared as
an integration constant.
