@phdthesis{Heuermann2009,
  author      = {Daniel Frederik Heuermann},
  title       = {Four Essays on Human Capital Externalities},
  type        = {doctoralthesis},
  school      = {Universit{\"a}t Trier},
  doi       = {10.25353/ubtr-xxxx-61ff-abf1},
  year        = {2009},
}