1. R. Hill, Mathematical Theory of Plasticity, Clarendon Press, Oxford (1956).
2. L. V. Ovsyannikov, Group Properties of Differential Equations [in Russian], Izd, AN SSSR, Sibirsk. Otd., Novosibirsk (1962).
3. B. D. Armin, ?New particular solutions of the three-dimensional problem of ideal plasticity,? in: Summary Reports of the All-Union Conference on the Application of the Theory of Limiting Equilibrium in the Statics and Dynamics of Thin-walled Three-Dimensional Structures [in Russian], Tbilisi (1971). (The full report will appear in the Conference Proceedings.)
4. D. D. Ivlev, Theory of Ideal Plasticity [in Russian], Nauka, Moscow (1966).