Exact Results on e+ e- --> e+ e- + 2 Photons at SLC/LEP Energies
  We use the spinor methods of the CALKUL collaboration, as realized by Xu, Zhang and Chang, to calculate the differential cross section for e+ e- --> e+ e- + 2 photons for c.m.s. energies in the SLC/LEP regime. An explicit complete formula for the respective cross section is obtained. The leading log approximation is used to check the formula. Applications of the formula to high precision luminosity calculations at SLC/LEP are discussed. 
S. Jadach, B.F.L. Ward and S.A. Yost