Comment on ``Photon Splitting in Strongly Magnetized Objects Revisited''
  I point out that the results stated in the recent articles on photon splitting by Wunner, Sang, and Berg and by Wentzel, Berg, \& Wunner directly contradict an earlier analytic and numerical calculation that I performed of the same process using Schwinger's proper time method, for strong magnetic fields and general energies below the pair production threshold. The results of Wunner et al. and Wentzel et al. do not show the expected low frequency behavior, nor have they been able to reproduce the expected small magnetic field behavior, suggesting that their calculations may not be gauge invariant. 
Stephen L. Adler