# Symmetry factors-Srednicki

1. Jan 17, 2009

### kexue

I am trying to understand Feynman diagrams and especially symmetry factors. I'm learning from Srednicki's QFT book page 60 to 63, or in the online version page 72 to 74.

Take formula 9.11 . (P-propagator, V-vertice, E-external source and E=2P-3V)

Srednicki says:
a, there are (2P)!/(2P-3V)! combinations that the 3V functional derivatives can act on the 2P sources

b, the number of terms for the RHS of 9.11 that result in one particular diagram is given by a -counting factor of 3! for each V
-a counting factor of V!
-a counting factor of 2! for each P
-a counting factor of P!

c, the counting factors cancel the number from the dual Taylor expansion in 9.11, but sometime we overcount the number of terms that give identical results, the factor by which we overcount is called symmetry factor

My questions
1. What is meant by statement b,? Do we multiply all these counting factors?
2. How do the (2P)!/(2P-3V)! combinations match the combinations that we got from drawing all the individual Feynam diagrams for a given E, P, V?
3. Do all the individual diagrams for a given E, P, V give the same number of possible terms for the RHS of 9.11? ( As by statement b, implied)
4. Why then different symmetry factors? How do they cancel the number from the dual Taylor expansion in 9.11?

My approach: -Pondering the above.
-I played for three hours with E=0, P=6, V=4 which corresponds to figure 9.2 and the diagrams showing there to see if something would make sense, but it did not.

Any help gratefully appreciated