Diagrams / Bare electron propagator

Bare electron propagator

Perturbative order	$\alpha$ (tree level)
Topology	tree
Symmetry factorCounts overcounting in the perturbation series. If a diagram has internal symmetries—ways to relabel internal lines and get the same diagram back—the symmetry factor divides that out. Most QED diagrams have S=1; the vacuum polarization bubble has S=1/2 because swapping the two internal fermion lines gives the same diagram.	1.0
Divergence	finite

Contributes to

Electron propagator

The free electron propagator before quantum corrections.

The computational chain

From diagram to number—every step of the amplitude calculation.

Feynman rule

$$\frac{i(\not{p}+m)}{p^2 - m^2 + i\epsilon}$$

The free electron propagator. The numerator $(\not{p}+m)$ is the spin completeness relation — it sums over intermediate spin states. The pole at $p^2 = m^2$ corresponds to an on-shell electron.