- #1
kof9595995
- 679
- 2
Equation (7.25)
[tex](\displaystyle{\not}p - m)(1 - {\left. {\frac{{d\Sigma }}{{d\displaystyle{\not}p}}} \right|_{\displaystyle{\not}p = m}}) + O({(\displaystyle{\not}p - m)^2})[/tex]
Formally it looks like a Taylor expansion of [itex]\displaystyle{\not}p-m_{0}-\Sigma(\displaystyle{\not}p)[/itex]. However it involves a differentiation of a matrix, and what's worse is, he let's [itex]\displaystyle{\not}p=m[/itex], which is impossible because [itex]\displaystyle{\not}p[/itex] is always off-diagonal(peskin uses weyl representaion), while m is diagonal.
The best I can make of this [itex]\displaystyle{\not}p=m[/itex] is that this is just a formal replacement, but then Taylor expansion loses its meaning of "polynomial approximation around the neighbourhood of a point", since [itex]\displaystyle{\not}p[/itex] can never really approach m.
[tex](\displaystyle{\not}p - m)(1 - {\left. {\frac{{d\Sigma }}{{d\displaystyle{\not}p}}} \right|_{\displaystyle{\not}p = m}}) + O({(\displaystyle{\not}p - m)^2})[/tex]
Formally it looks like a Taylor expansion of [itex]\displaystyle{\not}p-m_{0}-\Sigma(\displaystyle{\not}p)[/itex]. However it involves a differentiation of a matrix, and what's worse is, he let's [itex]\displaystyle{\not}p=m[/itex], which is impossible because [itex]\displaystyle{\not}p[/itex] is always off-diagonal(peskin uses weyl representaion), while m is diagonal.
The best I can make of this [itex]\displaystyle{\not}p=m[/itex] is that this is just a formal replacement, but then Taylor expansion loses its meaning of "polynomial approximation around the neighbourhood of a point", since [itex]\displaystyle{\not}p[/itex] can never really approach m.