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The arXiv documentation says that we should reference arXiv documents using references of the form:
arXiv:YYMM.NNNNv# [category]
e.g.:
arXiv:0706.1234v1 [math.FA]
However, most papers that I am aware of do not bother to include the category. i.e. they have references of the form:
arXiv:YYMM...