Applying the Factorization Technique for Simplification of Expressions Involving Differential Operator Inverses Serguei P. Tsarev Department of Mathematics Krasnoyarsk State Pedagogical University Lebedevoi, 89 660049, Krasnoyarsk, Russia e-mail: tsarev@edk.krasnoyarsk.su Using the formal theory of linear ordinary differential operators we develop simplification and normalization techniques for manipulation of expressions involving inverses of linear ordinary differential operators alongside with the usual D=d/dx. Applications to investigation of symmetries of nonlinear PDE are discussed.