The monodromy equation:

Set  and consider the monodromy equation


The behavior of solutions to (5) is very intimately related to the behavior of 
solutions to our initial quasi-periodic equation. 
One proves





The question  "What is the spectral nature of?" has turned into the question
compute the monodromy matrix and solve the monodromy equation.

To do this, we introduce a semi-classical parameter.