= One-particle-irreducible two-point vertex
{title2=$\widetilde\Gamma^{(2)}(p)=\widetilde G(p)^{-1}$}
The second derivative of a Legendre <effective action> is the inverse connected two-point kernel. In a translation-invariant scalar theory it is $\widetilde\Gamma^{(2)}(p)=\widetilde G(p)^{-1}$. Its loop expansion contains the reference inverse propagator, any separately displayed <counterterms>, and the <self-energy>. A generic amputated connected higher-point function is not the same as a <one-particle-irreducible vertex>.
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