  In bike sharing systems the quality of the service to the users strongly
depends on the strategy adopted to reposition the bikes. The bike repositioning
problem is in general very complex as it involves different interrelated
decisions: the routing of the repositioning vehicles, the scheduling of their
visits to the stations, the number of bikes to load or unload for each station
and for each vehicle that visits the station. In this paper we study the
problem of optimally loading/unloading vehicles that visit the same station at
given time instants of a finite time horizon. The goal is to minimize the total
lost demand of bikes and free stands in the station. We model the problem as a
mixed integer linear programming problem and present an optimal algorithm that
runs in linear time in the size of the time horizon.
