@article{KellerSchwarz2020,
  author    = {Matthias Keller and Michael Schwarz},
  title     = {Courant's nodal domain theorem for positivity preserving forms},
  journal   = {Journal of spectral theory},
  volume    = {10},
  number    = {1},
  pages     = {271 -- 309},
  doi       = {10.4171/JST/292},
  year      = {2020},
}