@book { EineempirischeTheoriedesZahlbegriffs,
    title = "Eine empirische Theorie des Zahlbegriffs",
    author = "Michael {Johann}",
    year = "1999",
    publisher = "Peter Lang Verlag",
    address = "Berlin, Germany",
    url = "https://www.peterlang.com/document/1088855"
}