A Complexity Theory for Hard Enumeration Problems
  Complexity theory provides a wealth of complexity classes for analyzing the complexity of decision and counting problems. Despite the practical relevance of enumeration problems, the tools provided by complexity theory for this important class of problems are very limited. In particular, complexity classes analogous to the polynomial hierarchy and an appropriate notion of problem reduction are missing. In this work, we lay the foundations for a complexity theory of hard enumeration problems by proposing a hierarchy of complexity classes and by investigating notions of reductions for enumeration problems. 
Nadia Creignou, Markus Kr\"oll, Reinhard Pichler, Sebastian Skritek,   Heribert Vollmer
cs.CC