@book { Businessplan,
    title = "Businessplan",
    subtitle = "Tools und Beratungsinstitutionen",
    author = "Gerhard {Humer}",
    year = "2002",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832453411",
    url = "https://m.diplom.de/document/220905"
}