Specifying a non-linear model

[KOLMOGOROV]

I apologize for my English low
I work on the determination of the optimal debt level for an economy X. in the literature that I have the authors work a lot more on that panel using panel models (the reference here is the Hansen model or quadratic equation in panel), however, I work on a country. can you have an article in any language that presents the methodology of modeling for a country in a clear way on my problematic.
Thank you in advance