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2. V. V. Dobronravov, Fundamentals of the Mechanics of Nonholonomic Systems [in Russian], Vysshaya Shkola (1970).
3. Bl. Dolapchiev, ?The Nielsen-Tsenov equations and their application to nonholonomic systems with nonlinear constraints,? Dokl. Akad. Nauk SSSR,171, No. 4 (1966).
4. Bl. Dolapchiev, ?Jourdain principle and the Nielsen equations? [in Bulgarian], Godishnik na Sofiiskiya Universitet, FMF,59, (1966).
5. Bl. Dolapchiev, ?The Appell-Tsenov forms of the equations of motion for holonomic and nonholonomic mechanical systems, method of generalization, and criteria for application? [in Bulgarian], Godishnik na Sofiiskiya Universitet, FMF,60 (1967).