Conclusion
Non-linear systems - First harmonic method

The method of first harmonic approximation is of considerable interest to engineers when studying looped systems with non-linearity. On the one hand, it can be used to evaluate permissible imperfections at the project level, and thus to specify components; on the other, it can be used to understand the behavior of an existing system, and thus to correct it.

Nevertheless, this remains an approximate method, and it is generally difficult to assess the error involved. It has even been shown that, in some cases, this method can lead to qualitatively erroneous conclusions (see 3.2