1. Basic ideas and concepts of differential geometry;D. Alekseevskij,1991
2. D. Hilbert, Theory of Algebraic Invariants (Cambridge Univ. Press, Cambridge, 1993).
3. V. V. Lychagin and V. A. Yumaguzhin, ‘‘Classification of second order linear differential operators and differential equations,’’ J. Geom. Phys. 130, 213–228 (2018).
4. V. V. Lychagin and V. A. Yumaguzhin, ‘‘On equivalence of third order linear differential operators on two-dimensional manifolds,’’ J.Geom. Phys. 146, 103507 (2019).
5. V. V. Lychagin and V. A. Yumaguzhin, ‘‘On structure of linear differential operators, acting in line bundles,’’ J. Geom. Phys. 148, 103549 (2020).