1. V. Z. Vlasov, Thin-Walled Three-Dimensional Systems [in Russian], Gosstroiizdat, Moscow (1958).
2. M. D. Dol'berg and N. N. Yasnitskaya, ?On a lower-bound estimate for the vibration frequencies of elastic system,? Dokl. Akad. Nauk SSSR,173, No. 3 (1967).
3. S. G. Lekhnitskii, Anisotropic Plates [in Russian], Gostekhizdat, Moscow (1957).
4. V. G. Tret'yak, ?Natural vibrations of orthotropic plates,? Prikl. Mekh.,2, No. 9 (1966).