1. Hua Loo Keng, Wu Ci-Quian and Lin Wei, The Systems of Partial Differential Equations of Second Order with Two Independent Variables and Two Unknown Functions, Science Press, Beijing (Peking) (1979).
2. Hua Loo Keng, Wu Ci-Quian and Lin Wei, On the classification of the system of differential equations of the second order. Scientia Sinica, Vol XIV, No. 3 (1964) pp. 461?465.
3. R.J. Knops and L.E. Payne, Uniqueness Theorems in Linear Elasticity, Springer, Berlin, Heidelberg, New York (1971).
4. S.G. Lekhnitskii, Anisotropic Plates, 2nd edn. Gordon and Breach, New York (1968).