“…Introduction and main results. During recent years the number of publications with the use of an involution operator in the theory of P T -symmetric operators, P T -symmetric quantum theory (see [11], [12]), various sections of the theory of ordinary differential equations (see [1], [4], [15], [17], [19], [21]- [24], [27], [31], [32]), partial differential equations (see [6], [10], [14], [20], [25]) and differential equations with operator coefficients (see [2], [3], [7], [8]) increased significantly.…”