Tensor product of modules over a vertex algebra
  We found a necessary and sufficient condition for the existence of the tensor product of modules over a vertex algebra. We defined the notion of vertex bilinear map and we provide two algebraic construction of the tensor product, where one of them is of ring theoretical type. We show the relation between the tensor product and the vertex homomorphisms. We prove the commutativity of the tensor product. We also prove the associativity of the tensor product of modules under certain necessary and sufficient condition. Finally, we show certain functorial properties of the vertex homomorphims and the tensor product. 
Jose I. Liberati
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