Positivity of Curvature-Squared Corrections in Gravity.

@article{Cheung2017PositivityOC,
  title={Positivity of Curvature-Squared Corrections in Gravity.},
  author={Clifford Cheung and Grant N. Remmen},
  journal={Physical review letters},
  year={2017},
  volume={118 5},
  pages={
          051601
        }
}
We study the Gauss-Bonnet (GB) term as the leading higher-curvature correction to pure Einstein gravity. Assuming a tree-level ultraviolet completion free of ghosts or tachyons, we prove that the GB term has a nonnegative coefficient in dimensions greater than 4. Our result follows from unitarity of the spectral representation for a general ultraviolet completion of the GB term. 
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