1. É. I. Grigolyuk and P. P. Chulkov, Stability and Vibrations of Three-Layer Shells [in Russian], Moscow (1973).
2. A. I. Smirnov, ?Vibrations of three-layer beams,? Dokl. Akad. Nauk SSSR,172, No. 3, 561?564 (1967).
3. Yu. D. Nasonkin, ?Natural vibrations of a three-layer rod for different attachment conditions,? in: The Dynamics of a Continuous Medium [in Russian], No. 6 (1970), pp. 111?116.
4. B. S. Ostapovich, ?An investigation of the effect of parameters on the lowest frequencies of three-layer beams,? Mat. Metody Fiz.-Mekh. Polya, No. 1, Resp. Mezhved. Sb. (1975), pp. 203?204.
5. L. I. Garmash, ?Approximate determination of the eigenfrequencies of three-layer rods under various end conditions,? in: The Strength of Materials and Structure Theory [in Russian], No. 25, Resp. Mezhved. Nauch.-Tekhn. Sb. (1975), pp. 130?136.