The article presents the method of the calculation of natural frequencies and mode shapes of the linear undamped rotationally periodic systems. The method is applied in the calculation of the natural vibration characteristics of the steam turbine bladed disks., Obsahuje seznam literatury, and Součástí článku je Appendix A - Solution of matrix difference equations
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To represent a set whose members are known partially, the graded ill-known set is proposed. In this paper, we investigate calculations of function values of graded ill-known sets. Because a graded ill-known set is characterized by a possibility distribution in the power set, the calculations of function values of graded ill-known sets are based on the extension principle but generally complex. To reduce the complexity, lower and upper approximations of a given graded ill-known set are used at the expense of precision. We give a necessary and sufficient condition that lower and upper approximations of function values of graded ill-known sets are obtained as function values of lower and upper approximations of graded ill-known sets.