I am reading the Lancaster & Blundell's Quantum field theory for the gifted amateur, p.183, Example 19.5 (Example of symmetry factors of several Feynman diagrams) and stuck at understanding several examples. I am self-studying quantum field theory and I can't possibly understand those calculations of symmetry factors on my own. Can anyone helps?
Previously, in his book, p.182, he wrote :
"The general rule is as follows : if there are $m$ ways of arranging vertices and propagators to give identical parts of a diagram ( keeping the outer ends of external lines fixed and without cutting propagator lines) we get a factor $D_i = m$. The symmetry factor if given by the product of all symmetry factors $D = \prod_{i} D_i$."
I don't understand this rule completely. Anyway, after he wrote
"Two very useful special cases, which aren't immediately apparent from the general rule, are as follows:
Every propagator with two ends joined to one vertex (a loop) gives a factor $D_i=2$.
Every pair of vertices directly joined by $n$ propagators gives a factor $D_i =n!$. "
I also don't understand this statement. Can anyone provides some explanation?
Anyway, examples of calculation of symmetry factors in his book that I stuck is as follows :
I can't understand the examples (e)~ (i). I somehow managed to understand (d) roughly, by refering How to count and 'see' the symmetry factor of Feynman diagrams?. But even if I try to find symmetry factors by imitating answer in the linked question, I won't be able to understand it and it will only add to confusion.
Can anyone explain the examples (e)~(i) by more step by step argument, if possible, using pictures ? I want to understand the 'all' examples completely.
For example, for (g), why its symmetry factor is $2$ , not $4$? And for (i), why is the symmetry factor only 2?
Understanding this issue will enhance understanding feynman diagram considerably. Can anyone helps?
EDIT : As a first attempt, in my opinion, I rougly guessed that
(e) Symmetric factor is $4$, since there are two loops. But in the Blundell's book, for the (e), he wrote that " (e) has a factor of $2$ from the bubble. It also has two vertices joined by two lines, contributing a factor $2!$" This statement gives me the impression that I guessed the value -by noting existence of two loops-the wrong way.
(f) Symmetric factor is $8$, since there are two loops and the two loops can be swapped ?
(g) Symmetric factor is $4$, since it seems that there are two loops
(h) I don't know why symmetry factor is $8$. It seems that there are three loops. Why the loop formed by the two external vertices is neglected?
(i) Similarly to (h), since it seems that there are two loops, symmetry factor is $4$.
Why the discrepancy occurs? Note that I am very begginer for symmetry factor. And please understanding my messy explanation and bad english.
Dreaming of complete understanding !