@book { GruppenundkomplexeProbleme,
    title = "Gruppen und komplexe Probleme",
    author = "Petra {Badke-Schaub}",
    year = "1994",
    publisher = "Peter Lang Verlag",
    address = "Berlin, Germany",
    url = "https://www.peterlang.com/document/1081845"
}