This paper is motivated by operating self service transport systems
that flourish nowadays. In cities where such systems have been set
up with bikes, trucks travel to maintain a suitable number of bikes
per station.
It is natural to study a version of the C-delivery TSP defined by
Chalasani and Motwani in which, unlike their definition, C is part
of the input: each vertex v of a graph G=(V,E) has a certain
amount xv of a commodity and wishes to have an amount equal to
yv (we assume that $\sum_{v\in V}x_v=\sum_{v\in V}y_v$ and all
quantities are assumed to be integers); given a vehicle of capacity
C, find a minimal route that balances all vertices, that is,
that allows to have an amount yv of the commodity on each vertex
v.
This paper presents among other things complexity results, lower
bounds, approximation algorithms, and a polynomial algorithm when
G is a tree.