SGEM 2019 : 19th international multidisciplinary scientific geoconference SGEM 2019, 30 June - 6 July, 2019, Albena, Bulgaria. 19, Ecology, economics, education and legislation. 1st ed. Sofia : STEP92 Technology, 2019. 1012 s. Available on Internet: <http://www.sgem.org> ISBN 978-619-7408-86-7.