@book { MakeorBuyEntscheidungimTravelManagement,
    title = "Make-or-Buy Entscheidung  im Travel Management",
    author = "Nicola {Hoffmann}",
    year = "2004",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832480844",
    url = "https://m.diplom.de/document/223312"
}