@book { DieHelmsdorferGruppe,
    title = "Die Helmsdorfer Gruppe",
    author = "Ernst {Probst}",
    year = "2011",
    publisher = "GRIN Verlag",
    address = "München, Germany",
    doi = "10.3239/9783656070627",
    url = "https://www.hausarbeiten.de/document/182515"
}