Numerical Methods and Applications 9th International Conference, NMA 2018, Borovets, Bulgaria, August 20-24, 2018, Revised Selected Papers - Nikolov, Geno; Nikolov, Geno; Kolkovska, Natalia; Kolkovska, Natalia; Georgiev, Krassimir; Georgiev, Krassimir (Springer International Publishing : Imprint: Springer, 2019)