Time-dependent deformations of powers of the wave operator

Conditions under which the linear differential operators of the second order are equivalent to operators not containing “friction” (first partial derivatives) are investigated. One can construct iso-Huygens deformations for powers of the wave operator with time-dependent coefficients. The fundamental solutions of these deformations and conditions under which the Huygens principle holds are found. Bibliography: 17 titles. __________ Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 324, 2005, pp. 213–228.