@book { LocationBasedServicesinDeutschland,
    title = "Location Based Services in Deutschland",
    author = "Ingolf Christian {Ernst}",
    year = "2003",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832465971",
    url = "https://m.diplom.de/document/221988"
}