“…During recent years, the number of publications with the use of an involution operator in various sections of the theory of ordinary differential equations (see [4,10,12,15,16,19]), of partial differential equations (see [3,7,9,14,16,17,20,21]), of linear operators, T-invariant with respect to some group of homeomorphisms (see [8]), differential equations with operator coefficients (see [5][6][7]), PT-symmetric operators (see [1,2]) increased significantly.…”