@article{Baer2015,
  author    = {Christian B{\"a}r},
  title     = {Geometrically formal 4-manifolds with nonnegative sectional curvature},
  journal   = {Communications in analysis and geometry},
  volume    = {23},
  number    = {3},
  pages     = {479 -- 497},
  year      = {2015},
}