• https://lib.uni-plovdiv.net/handle/123456789/1
• https://lib.uni-plovdiv.net/handle/123456789/4
• https://lib.uni-plovdiv.net/handle/123456789/2
• https://lib.uni-plovdiv.net/handle/123456789/5
• https://lib.uni-plovdiv.net/handle/123456789/3
• https://lib.uni-plovdiv.net/handle/123456789/6
• https://lib.uni-plovdiv.net/handle/123456789/7