Hello there, First of all, I am not fully sure that this is the right place to post this. This post is about equations but the paper is a bout physics... so if an admin find a better place for it please feel free to move it. I am currently trying to fully understand this paper http://link.springer.com/article/10.1007/BF01091215 I hope some people have acces to it because I don't think putting only the equation I don't understand is usefull. So on that paper, I went through all the equations to be sure I understand how he goes from one to another. However, I can't manage to go from A3 to A4. Could someone help me with that ? He does some simplifications which I tried to use but I am allways ending up stucked. Thanks in advance for your help. PS: please forgive my english. It is not my native language and I am not fully comfortable in writting it.