Feynman rules

Vertices and propagators

$feyn$	$= -i e \gamma^\mu\,$ ^[1]
$feyn$	$= \frac{-i}{p^2} \left( g^{\mu\nu} - (1-\xi) \frac{p^\mu p^\nu}{p^2} \right)\,$ ,
$feyn$	$= \frac{i}{p\!\!\!/ - m} = i\frac{p\!\!\!/ + m}{p^2 - m^2}\,$ ^[2]

↑ The minus sign arises because the electron has charge $-1\,$ .
↑ The propagator is written assuming that $p\,$ flows in the direction of the arrow