@book { AltersversicherunginDeutschland,
    title = "Altersversicherung in Deutschland",
    author = "Christian {van Heynsbergen}",
    year = "2005",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832440640",
    url = "https://m.diplom.de/document/219693"
}