@book { VirtuelleUniversitten,
    title = "Virtuelle Universitäten",
    author = "Silvia {Friedrich}",
    year = "2000",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832428976",
    url = "https://m.diplom.de/document/218623"
}