@article{SchmidtSingleton2013,
  author    = {Hans-J{\"u}rgen Schmidt and Douglas Singleton},
  title     = {Isotropic universe with almost scale-invariant fourth-order gravity},
  journal   = {Journal of mathematical physics},
  volume    = {54},
  number    = {6},
  doi       = {10.1063/1.4808255},
  year      = {2013},
}