A fence needs to be guarded to prevent intruders from crossing. To make sure that no intruder has time to cross, each point of the fence needs to be visited by a guard at least once every unit of time. There are $k$ guards and each guard has maximum speed $v_i$ ($1 \leq i \leq k$). What is the longest fence that can be guarded?
In this talk, I will present a survey of the results related to this topic.