@incollection{Appell1986,
  author    = {J{\"u}rgen Appell},
  title     = {G. Darbo's fixed point principle after 30 years},
  booktitle = {Nonlinear functional analysis and its applications},
  editor    = {S. P. Singh},
  isbn      = {978-90-277-2211-9},
  doi       = {10.1007/978-94-009-4632-3_8},
  year      = {1986},
}