Authors
Snezhana G. Gocheva-Ilieva
Faculty of Mathematics and Informatics, University of Plovdiv Paisii Hilendarski, Plovdiv, Bulgaria
Atanas V. Ivanov
Faculty of Mathematics and Informatics, University of Plovdiv Paisii Hilendarski, Plovdiv, Bulgaria
Ioannis E. Livieris
Department of Mathematics, University of Patras, Patras, Greece
