**9. Conclusions**

In this work, we proposed three different approaches for estimating the quadratic forms of the type *<sup>x</sup>TA*−*mx*, *m* ∈ N. Specifically, we considered a projection method, a minimization approach, and a heuristic procedure. We also expressed upper bounds on the absolute error of the derived estimates; they allowed us to assess the precision of the results obtained by the aforementioned methods.

The proposed approaches provide efficient and fast estimates. Their efficiency was illustrated by numerical examples. Comparing the proposed estimates with the corresponding ones presented in the literature, we formed the following conclusions.


**Author Contributions:** Conceptualization, M.M., P.R., and O.T.; methodology, M.M., P.R., and O.T.; software, A.P. and P.R.; validation, M.M., P.R., and O.T.; formal analysis, A.P. and P.R.; investigation, M.M., A.P., P.R., and O.T.; data curation, A.P. and P.R.; writing—original draft preparation, M.M., P.R., and O.T.; writing—review and editing, M.M., P.R., and O.T.; visualization, A.P. and P.R.; supervision, M.M.; project administration, M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** We thank the reviewers of the paper for the valuable remarks. This paper is dedicated to Constantin M. Petridi.

**Conflicts of Interest:** The authors declare no conflict of interest.
