What are applications of the theory of differential equations in complex domain in physical sciences or engineering? In which physical science differentiation wrt complex variable makes sense?
The theory of diferential equations in complex domain is very well developed and is one the most beautiful areas of mathematics. A well known old book on this subject is by E. Hill. There are many contemporary results in this area but all of them I found are in mathematics. I would like to know a physical science or engineering application.