Authors
Ivaylo Vladev
Konstantin Preslavsky University of Shumen, Faculty of Natural Sciences, Shumen, Bulgaria
Rositsa Vladeva
Konstantin Preslavsky University of Shumen, Faculty of Natural Sciences, Shumen, Bulgaria
© 2020 Ivaylo Vladev, Rositsa Vladeva, published by Konstantin Preslavski University of Shumen
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.