1. University of Architecture, Civil Engineering and Geodesy, Hr. Smirnenski Blvd. 1, 1046 Sofia, Bulgaria
2. University of Forestry, Kl. Ohridsky Blvd. 10, 1756 Sofia, Bulgaria
3. Faculty of Physics, Sofia University “St. Kliment Ohridski”, J. Bouchier Blvd. 5, 1164 Sofia, Bulgaria