One-particle-irreducible vertex
= One-particle-irreducible vertex
{title2=$\Gamma^{(n)}$}
A <functional derivative> of the <quantum effective action> gives an amputated <one-particle-irreducible correlation function>. Its perturbative graphs stay connected after cutting any single internal propagator. A free quadratic effective <action> has no vertices of order greater than two; interactions generate higher vertices.