@book { DasDreiSchichtenModellderAltersvorsorge,
    title = "Das Drei-Schichten-Modell der Altersvorsorge",
    author = "Tobias {Niedrig}",
    year = "2009",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836626071",
    url = "https://m.diplom.de/document/226559"
}