In this answer Proof of geometric series two-point function it is said:
- Now what about the coefficients in front of each Feynman diagram? Due to the combinatorics/factorization involved it becomes a geometric series $$G_c~=~G_0\sum_{n=0}^{\infty}(\Sigma G_0)^n\tag{A}$$
How can we prove this? My main concern is the combinatorics, for example in qed the symmetry factor for connected diagrams is 1.suppose that $\Sigma=A+B+...$ are irreducible diagrams of the photon propagator.
Since symmetry factor of $G_0AG_0AG_0=1$ we should have the symmetry factor of $A=1$.The same thing for $B$.
But we also have the factor $G_0AG_0BG_0+G_0BG_0AG_0$
The only way this to work is that $G_0AG_0BG_0=-G_0BG_0AG_0$
How can I prove this?