6. Conclusion
In Refs. [1–3] it has been shown, that the infinite system of .... (21) (if one supposes ...) cannot be
reduced. On the other hand, in engineering papers (see the references in Refs. [1–3]) only a few terms in the
series (14) will usually be used. In what way can this contradiction be resolved?
It should be clear that the applicability of the different models is related to the frequencies (for the lower
frequencies a string-like approach can be used, and for the higher frequencies stretched beam or beam-like
approaches should be used). The different models still can (or should) be linked. The combination of models is
new; and in the paper it is indicated how this method (of different models for different frequencies domains)
can most likely be applied.
In mechanics usually a hierarchy of models can be used. In our case (depending on the frequency domain)
there are string, stretched beam, beam, and 3D elasticity models. In turn, the applicability of a 3D model will
be restricted by the atomic structure of the material. So, each model can only be used for a finite range of the
parameters.
At the end of Section 3 it has explicitly been assumed that the string model for the modes ...., the
stretched beam model for the modes..., and the beam model for the modes ... are not
interacting. However, by putting