@book { Lernen,
    title = "Lernen",
    author = "Karin {Kröninger} and Thomas {Pietzsch}",
    year = "2014",
    publisher = "Peter Lang Verlag",
    address = "Berlin, Germany",
    doi = "10.3726/978-3-653-03643-5",
    url = "https://www.peterlang.com/document/1048132"
}