The work of D.Treschev was supported by the Russian Science Foundation under grant no. 20-11-20141, https://rscf.ru/en/project/20-11-20141/. The work of S.Bolotin was performed at the Steklov International Mathematical Center and supported by the Ministry of Science and Higher Education of the Russian Federation (agreement no. 07515-2022-265). Sections 1, 2, 4, 5.6, 6.1, 6.2 were written by S. Bolotin, and sections 3, 5.1, 5.2, 5.3, 5.4, 5.5, 7 by D. Treschev.