This paper analyses the construction of the kernel graph of a non-synchronizing transformation semigroup and introduces the inverse synchronization problem. Given a transformation semigroup S ≤ Tn, we construct the kernel graph Gr(S) by saying v and w are adjacent, if there is no f ∈ S with vf = wf . The kernel graph is trivial or complete if the semigroup is a synchronizing semigroup or a perm...

We define the notion of a torsor for an inverse semigroup, which is based on semigroup actions, and prove that this is precisely the structure classified by the topos associated with an inverse semigroup. Unlike in the group case, not all set-theoretic torsors are isomorphic: we shall give a complete description of the category of torsors. We explain how a semigroup prehomomorphism gives rise t...