1. Karnopp, B. H.,Duality Relations in the Analysis of Beam Oscillations, Z. angew. Math. Phys.18, 575 (1967).
2. Timoshenko, S.,Vibration Problems in Engineering, Third Edition, D. Van Nostrand, New York (1955), pp. 329–331.
3. Huang, T. C.,Eigenvalues and Modifying Quotients of Vibrations of Beams, U. of Wisconsin Eng'g. Exper. Station Report No.25, (1964), pp. 2, 5.
4. Crandall, S. H.,Complementary Extremum Principles for Dynamics, Ninth International Congress of Applied Mechanics, Brussels (1956), p. 86.
5. Reissner, E.,Note on the Method of Complementary Energy, J. Math. Phys.27, 159–160 (1948).