@book { ExistenzsicherungeinesReisebros,
    title = "Existenzsicherung eines Reisebüros",
    author = "Madlen {Röhler}",
    year = "2007",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836603287",
    url = "https://m.diplom.de/document/228902"
}