I am answering a question about a particle decaying into two other particles and one needs to find the momentum of the children particles from the rest frame of the parent. I found a very helpful article on Wikipedia under 'Particle Decay', however I am really confused by their mathematical working and wondering if anyone can explain it or agree with me that Wiki has made a mistake :) I can't copy all the equations so the link is here: http://en.wikipedia.org/wiki/Particle_decay its under the Sub heading Conservation of four momentum. I don't understand how they can expand 2(p_M*p_1) into only 2 terms when under normal (a+b)(c+d) expansions you get four. Unless there is some wierd vector multiplication I don't understand? Any help GREATLY appreciated!