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4. I. Tadjbakhsh and J.B. Keller, Strongest columns and isoperimetric inequalities for eigenvalues, J. of Applied Mechanics, vol. 9, (1962), pp. 159–164.
5. N. Olhoff, Optimal design against structural, vibration, and instability, Ph.D. Thesis, Technical University of Denmark, Dept. of Solid Mechanics, Lyngby, Denmark, November, 1978.