@book { DiegesetzlicheRentenversicherung,
    title = "Die gesetzliche Rentenversicherung",
    author = "Jens {Katzenberger}",
    year = "1998",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832411039",
    url = "https://m.diplom.de/document/216993"
}