1. https://whc.unesco.org/en/statesparties/bg
2. R. C. Martin, Clean Architecture: A Craftsman's Guide to Software Structure and Design (Pearson, 2017)
3. V. Lee, Mobile Applications: Architecture, Design, and Development: Architecture, Design, and Development (Prentice Hall, 2004)
4. C. Jones, Software Methodologies: A Quantitative Guide, (CRC Press, 2017)
5. I. Sommerville, Software Engineering (10th Edition) (Pearson, 2015)