@book { DurchstartenalsFreiberufler,
    title = "Durchstarten als Freiberufler",
    author = "Martin {Buttenmüller} and Claudia {Kilian}",
    year = "2014",
    publisher = "Vahlen",
    address = "München, Deutschland",
    url = "https://vahlen.openpublishing.com/document/1220799"
}