@book { DieOsterweiterungderEuropischenUnion,
    title = "Die Osterweiterung der Europäischen Union",
    author = "Tom {Heger}",
    year = "2001",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832447380",
    url = "https://m.diplom.de/document/220337"
}