From a paper I've been reading: I wanted to know how to interpret the notation. What does it mean to take the derivative of u when it is a function argument? How do I take the integral wrt x of a seemingly-constant expression and not end up with x in the answer? Generally, how are these functions (typically defined with definite integrals) defined with indefinite integrals? (Or is this just a trick of notation?) I'd be happy to post more of the paper if it would help. I'm actually recopying it at the moment, so it wouldn't be too hard.