Logarithmic tensor category theory, VII: Convergence and extension   properties and applications to expansion for intertwining maps
  This is the seventh part in a series of papers in which we introduce and develop a natural, general tensor category theory for suitable module categories for a vertex (operator) algebra. In this paper (Part VII), we give sufficient conditions for the existence of the associativity isomorphisms. 
Yi-Zhi Huang, James Lepowsky and Lin Zhang
math.QA hep-th math.RT