What are the key concepts in Srednicki's explanation of Feynman diagrams in QFT?

[kexue]

I see.

d(x1)d(x2)d(x3)d(w1)d(w2)d(w3) J(y1)J(z1)J(y2)J(z2)J(y3)J(z3)=
d(x1)d(x2)d(x3)d(w1)d(w2)[J(y1)J(z1)J(y2)J(z2)J(y3)Delta(w3-z3) +J(y1)J(z1)J(y2)J(z2)Delta(w3-y3)J(z3)+...]
=d(x1)d(x2)d(x3)d(w1)[J(y1)J(z1)J(y2)J(z2)Delta(w2-y3)Delta(w3-z3) +d(x1)d(x2)d(x3)d(w1)[J(y1)J(z1)J(y2)Delta(w2-z2)J(3)Delta(w3-z3)+..+ J(y1)J(z1)J(y2)J(z2)Delta(w3-y3)Delta(w2-z3)+...]= and so on

makes 6! terms

thank you!

 

[Avodyne]

Yes. But just to be absolutely clear on the notation, the delta's on the right-hand side are Dirac delta functions \delta^4(x-y) and not Feynman propagators \Delta(x-y).

 

[kexue]

I was celebrating the last days that I had finally understood 9.11.

But as read on just two pages later, Srednicki hit me with it. Out of blue he is claiming, that what we have so far computed, which happened to be connected diagrams, are not the only contributions to Z(J). We also have to take account of products of several connected diagrams.

How can that be? When I look at 9.11, from where in the world should the need arise to form products of several connected diagrams?

thanks

 

[kexue]

After some thought I see now what he means.

Silly question!

 

[RedX]

I found a website that discusses the chapter you're on, though I'm not sure if it'll be helpful:

http://www.physics.indiana.edu/~dermisek/QFT/qft-II-1-4p.pdf

 

[kexue]

Great find, RedX!

Glad to know that you like the book, too.

I'm very soon entering the chapters on renormalization. Even though I try to take Srednicki's advice 'reading passages you don't get four times' more to heart, I will surely turn to physics forums for some more help.