A remark on contracting inverse semigroups
  A semi-lattice is said to be tree-like when any two of its elements are either orthogonal or comparable. Given an inverse semigroup S whose idempotent semi-lattice is tree-like, and such that all tight filters are ultra-filters, we present a necessary and sufficient condition for S to be contracting which looks closer in spirit to the notion of contracting actions than a recent condition found by the second named author and E. Pardo. 
Giuliano Boava and Ruy Exel
math.OA math.CO