In this forum, I often post amazon links to specific scientific books. But recently, as in this post https://www.physicsforums.com/threads/what-is-method-of-characteristics.828118/#post-5225113 I have noticed that a link gets automatically changed to another (non-amazon) link, which is rather useless. Why does it happen? Is it beacuse it is forbidden to advertize amazon? What can I do about that?