Comments on "Translation-invariant bipolarons and the problem of   high-temperature superconductivity"
  We comment on the recent results of Refs. [1, 2] on the bipolaron problem derived using an approximation of Gross - Tulub. It is proved that, contrary to the claim made in Refs. [1, 2], the bipolaron ground state energy calculated there in the strong-coupling approximation has not been shown to constitute a variational upper bound. 
S. N. Klimin and J. T. Devreese
cond-mat.other