@book { DerMobilfunkmarktinDeutschland,
    title = "Der Mobilfunkmarkt in Deutschland",
    author = "Gabi {Sammut}",
    year = "2007",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836604161",
    url = "https://m.diplom.de/document/228934"
}