Analytic Continuation – Nevanlinna method
The Nevanlinna Analytic Continuation method is described in detail in the following two papers:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.056402
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.165111
There is also an undergraduate thesis that has many more details: https://deepblue.lib.umich.edu/handle/2027.42/167885
Jiani Fei gave a talk on how to use the Nevanlinna code and the mathematical background:
Here are her slides.
Note that the talk mentions source files for the optimization procedure. You can find those here.