  This is the sixth 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 VI), we
construct the appropriate natural associativity isomorphisms between triple
tensor product functors. In fact, we establish a "logarithmic operator product
expansion" theorem for logarithmic intertwining operators. In this part, a
great deal of analytic reasoning is needed; the statements of the main theorems
themselves involve convergence assertions.
