In Peskin's QFT book chapter 11.5, the author gives a graph(see below) and claims that the third functional derivative for effective action $\Gamma$ gives the 1PI (one-particle-irreducible) 3-point Greens's function.
When computing the electron propagtor, I learned that the full electron propagtor is a geometric series of 1PI bubble(1PI bubble=sum of all 1PI diagrams).
So how can I see that the white bubble is the 1PI 3-point Green's function? Is it a geometric series of something? What does the white bubble contain?
