1. Saratov State University, 83, Astrakhanskaya st., Saratov 410012, Russia
2. Tsinghua - Berkeley Shenzhen Institute, Building C2&C3, Zhiyuan, No. 1001 Xueyuan Avenue, Nanshan District, Shenzhen, P. R. China
3. Tomsk State University, 36, Lenin Avenue, Tomsk 634050, Russia