# Soft-gluon of scattering amplitude

The question from Scattering Amplitudes in Gauge Theory and Gravity page 46, equation (2.120) from which I can't get a consistent formula.

I just calculate A3(gff) which two fermion and one gluon $$A_n\left[\ldots,f^+_{i-1},i^−,f^-_{i+1},\ldots\right]$$ sign show helicity of particle.

Use Feynman diagram of qed soft-photo from Peskin page 182 equation (6.20).

I get formula of the first term of (2.120) as $$-\frac{\langle i+1,i \rangle[q,i-1]}{[qi] s_{i,i+1}}$$ instead of the one shown below.

Which is the correct formula?

• Elvang anf Huang's metric convention is mostly positive, P$\&$S is mostly negative. Have you taken care of this? – Oбжорoв May 1 at 12:06