2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/57359New concept of conditional differential invariant is discussed that would allow description of equations invariant with respect to an operator under a certain condition. Example of conditional invariants of the projective operator is presented.8 pages; based on talk at "Symmetry in Nonlinear Mathematical Physics", Kiev 2001Mathematical PhysicsAnalysis of PDEs35A20; 35L70; 58G35text