@book { RudolfMichael,
    title = "Rudolf Michael",
    author = "Christian {Sonntag}",
    year = "2014",
    publisher = "Peter Lang Verlag",
    address = "Berlin, Germany",
    doi = "10.3726/978-3-653-04037-1",
    url = "https://www.peterlang.com/document/1048330"
}