Bolgarskie zemli v 6-4 tysjačeletii do n.ė. i nekotorye voprosy drevnejšej istorii Evropy. Bulgarian lands in the 6th-4th millennia B.C. and some problems of ancient European history - Merpert, Nikolaj Jakovlevič (Peterburgskij Archeologičeskij Vestnik, 9 (1994), 1994)