[1] Fadell E. R., Husseini S. Y., Rabinowitz P. H.:
Borsuk-Ulam theorems for arbitrary $S^1$ actions and applications. Trans. A.M.S. 274 (1982), pp. 345-360.
MR 0670937
[2] Feireisl E.:
Free vibrations for an equation of a rectangular thin plate. to appear in Aplikace matematiky.
MR 0940708
[5] Štědrý M., Vejvoda O.:
Existence of classical periodic solutions of a wave equation: a connection of a number-theoretical character of the period with geometrical properties of solutions. (in Russian), Differencialnye uravnenia 20 (1984), pp. 1733-1739.
MR 0767883