Abstract: The volume is dedicated to the 70th birthday of Armen Glebovich Sergeev. It contains works related to the diverse scientific interests of A. G. Sergeev in mathematical physics, spectral theory, real and complex analysis, and complex differential geometry. The results of some of the articles were presented at the Conference on Complex Analysis and Mathematical Physics (Moscow, Steklov Math. Inst., March 18–22, 2019) and at the VIII Russian–Armenian Workshop on Mathematical Physics, Complex Analysis and Related Topics (Moscow, Steklov Math. Inst., September 16–20, 2019).