@book { BulgarienJahrbuch2013,
    title = "Bulgarien-Jahrbuch 2013",
    author = "Helmut {Schaller} and Sigrun {Comati} and Raiko {Krauß}",
    year = "2015",
    publisher = "Biblion",
    address = ", Germany",
    doi = "10.3726/b12811",
    url = "https://www.peterlang.com/document/1067653"
}