@book { Kerschensteiner,
    title = "Kerschensteiner",
    author = "Sonia {Fertinger}",
    year = "2001",
    publisher = "GRIN Verlag",
    address = "München, Germany",
    doi = "10.3239/9783638973113",
    url = "https://www.hausarbeiten.de/document/98860"
}