The concept of reload cost, that is of a cost incurred
when two consecutive arcs along a path are of different
types, naturally arises in a variety of applications
related to transportation, telecommunication, and energy
networks. Previous work on reload costs is devoted
to the problem of finding a spanning tree of minimum
reload cost diameter (with no arc costs) or of minimum
reload cost. In this article, we investigate the complexity
and approximability of the problems of finding
optimum paths, tours, and flows under a general cost
model including reload costs as well as regular arc costs.
Some of these problems, such as shortest paths and
minimum cost flows, turn out to be polynomially solvable
while others, such as minimum shortest path tree
and minimum unsplittable multicommodity flows, are
NP-hard to approximate within any polynomial-time computable
function