@book { ProfessionalsRealEstate,
    title = "Professionals Real Estate",
    author = "Annegret {Schacks}",
    year = "2003",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783832474003",
    url = "https://m.diplom.de/document/222686"
}