“…Equation (19) appeared in the context of bicycle kinematics in [11,41] and in the papers by Wegner, summarized in [47]. It also appeared in [27] in the context of billiards and flotation problems, and in [8], [9], [10] for magnetic, outer and wire billiards. This ubiquitous equation has a countable number of solutions but, except for đťś‹/2, there are no đťś‹-rational solutions [18].…”