yup...if you look at equation (24). in the limit t ---> infinity, the term inside the square brackets --> 1, so x approaches x_0, which is the line that B will always be on. In order to catch B, a necessary condition for A is to have an x coordinate that equals B's x coordinate. This occurs only in the limit, so A never catches B. bisho, i did not answer your question directly, but your question made me think about something else. so kudos for that.