  For the directed edge preferential attachment network growth model studied by
Bollobas et al. (2003) and Krapivsky and Redner (2001), we prove that the joint
distribution of in-degree and out-degree has jointly regularly varying tails.
Typically the marginal tails of the in-degree distribution and the out-degree
distribution have different regular variation indices and so the joint regular
variation is non-standard. Only marginal regular variation has been previously
established for this distribution in the cases where the marginal tail indices
are different.
