  We study the mechanism design problem of scheduling unrelated machines and we
completely characterize the decisive truthful mechanisms for two players when
the domain contains both positive and negative values. We show that the class
of truthful mechanisms is very limited: A decisive truthful mechanism
partitions the tasks into groups so that the tasks in each group are allocated
independently of the other groups. Tasks in a group of size at least two are
allocated by an affine minimizer and tasks in singleton groups by a
task-independent mechanism. This characterization is about all truthful
mechanisms, including those with unbounded approximation ratio.
  A direct consequence of this approach is that the approximation ratio of
mechanisms for two players is 2, even for two tasks. In fact, it follows that
for two players, VCG is the unique algorithm with optimal approximation 2.
  This characterization provides some support that any decisive truthful
mechanism (for 3 or more players) partitions the tasks into groups some of
which are allocated by affine minimizers, while the rest are allocated by a
threshold mechanism (in which a task is allocated to a player when it is below
a threshold value which depends only on the values of the other players). We
also show here that the class of threshold mechanisms is identical to the class
of additive mechanisms.
