@book { PricecompetitionattheGermanmobilemarket,
    title = "Price-competition at the German mobile market",
    author = "Volker {Küpper}",
    year = "2007",
    publisher = "GRIN Verlag",
    address = "München, Germany",
    doi = "10.3239/9783638719322",
    url = "https://www.hausarbeiten.de/document/77306"
}