@book { DerCustomerLifetimeValue,
    title = "Der Customer Lifetime Value",
    author = "Matthias {Dehn}",
    year = "2001",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832445720",
    url = "https://m.diplom.de/document/220152"
}