@book { DieDeutscheRentenversicherung,
    title = "Die Deutsche Rentenversicherung",
    author = "Marco {Zimmer}",
    year = "2005",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832488024",
    url = "https://m.diplom.de/document/223982"
}