@book { Investitioninbungen,
    title = "Investition in Übungen",
    author = "Hartmut {Bieg} and Heinz {Kußmaul} and Gerd {Waschbusch}",
    year = "2020",
    publisher = "Vahlen",
    address = "München, Deutschland",
    url = "https://vahlen.openpublishing.com/document/1221384"
}