Express_as_exponent​ial_series

This function is designed to convert a general expression into a sum of coefficients with exponential terms. This requires the use of the symbolic toolbox and is based mainly on character string logic, which may break down for complicated expressions.
The main motivation for such a series expression is to allow for the identification of fast rotating terms (applying a generalised rotating wave type approximation) and fast decaying terms in analytic time-correlation functions. This is also a very simple form for taking derivatives.