Feynman Diagram 4

Dressed four-point vertex $\Gamma^{(4)}_{k,abcd}(p_1,p_2,p_3,p_4)$ joining four external fields, drawn on axes rotated by 45° so the legs run diagonally. Vertices like this are why the flow equations form an infinite tower: the scale derivative of any $n$-point function depends on the $(n+1)$- and $(n+2)$-point functions, so the hierarchy has to be truncated.