vertex3 from the parquet equation may need an additional sign

[kunyuan]

The one-loop vertex3 calculated from the following source code gives F = -0.5123(18). Since z factor is expected to be 0.662, so that we expect F = 1/z-1=0.51057. It indicates an additional sign is needed for the fermionic vertex correction.

https://github.com/numericalEFT/FeynmanDiagram.jl/blob/81a7fa415eb90454057662f53d9d1ad613cf7cdb/example/vertex4/ver3_l.jl

[kunyuan]

Since we define Gamma3 = (G^{-1} (k1)- G^{-1} (k2))/(k1-k2), and G^{-1}=g^{-1}-\Sigma
gamma3 is expected to have this additional sign compared to Sigma.
The problem doesn't exist.