Adjoint Functors, Projectivization, and Differentiation Algorithms for   Representations of Partially Ordered Sets
  Adjoint functors and projectivization in representation theory of partially ordered sets are used to generalize the algorithms of differentiation by a maximal and by a minimal point. Conceptual explanations are given for the combinatorial construction of the derived set and for the differentiation functor 
Mark Kleiner and Markus Reitenbach
math.RT math.RA