\lim_{\zeta\to0}
\feynmandiagram [small, baseline=(a.base), horizontal=a to c, layered layout] {
a -- [double fermion, edge label'=$c$] b [semi hollow triangle] -- [double fermion, edge label'=$c$] c,
b -- [anti boson fermion, out=-135, in=-45, loop, min distance=2cm, edge label=$Q$] b
};
=
\lim_{\zeta\to0}
\feynmandiagram [small, baseline=(a.base), horizontal=a to c, layered layout] {
a -- [double fermion, edge label'=$c$] b [triangle] -- [double fermion, edge label'=$c$] c,
b -- [anti boson fermion, out=-135, in=-45, loop, min distance=2cm, edge label=$Q$] b
};
\stackrel{\mathrm{(NCA)}}{=}
\lim_{\zeta\to0}
\feynmandiagram [small, baseline=(a.base), horizontal=a to d, layered layout] {
a -- [double fermion, edge label'=$c$] b [dot, label=$V_0^*$] -- [double charged scalar, half left, edge label'=$f$] c [dot, label=$V_0$] -- [double fermion, edge label'=$c$] d,
b -- [double anti charged boson, half right, edge label=$b$] c,
};
% \times
% \ev{\hat Q}_\zeta
% + ...