David Tong‘s lecture notes are pretty good: What I am talking about is covered in lecture 7: http://www.damtp.cam.ac.uk/user/tong/string/seven.pdf http://www.da

David Tong‘s lecture notes are pretty good: What I am talking about is covered in lecture 7: http://www.damtp.cam.ac.uk/user/tong/string/seven.pdf http://www.damtp.cam.ac.uk/user/tong/string/seven.pdf