  For the problem of delivering a package from a source node to a destination
node in a graph using a set of drones, we study the setting where the movements
of each drone are restricted to a certain subgraph of the given graph. We
consider the objectives of minimizing the delivery time (problem DDT) and of
minimizing the total energy consumption (problem DDC). For general graphs, we
show a strong inapproximability result and a matching approximation algorithm
for DDT as well as NP-hardness and a 2-approximation algorithm for DDC. For the
special case of a path, we show that DDT is NP-hard if the drones have
different speeds. For trees, we give optimal algorithms under the assumption
that all drones have the same speed or the same energy consumption rate. The
results for trees extend to arbitrary graphs if the subgraph of each drone is
isometric.
