@book { DieOsterweiterungderEuropischenUnion,
    title = "Die Osterweiterung der Europäischen Union",
    author = "Stefania {Slavu}",
    year = "2021",
    publisher = "Peter Lang Verlag",
    address = "Berlin, Germany",
    url = "https://www.peterlang.com/document/1105375"
}