I am reading a paper about optimsation and the description, while mostly being a very good description, makes reference to some variables being in some set $E$. For example, it states that parameter $x\in E^n$. However it does not mention the significance of $E$ or why it did not say $\mathbb{R}$ instead. I suppose the algorithm in discussion doesn't only apply to reals, but also complex numbers, etc. I am wondering though, whether the description merely means $E$ to mean, "some set," or if they mean something more specific. For example it could mean E to stand for "enumerated," which I guess would mean they only mean to refer to "computer-representable" numbers, which I guess technically is a subset of $\mathbb{R}$. I am not familiar with any special significance of the letter $E$ in engineering as applied to sets, so I wanted to make sure by asking.
Here is the description from the paper:
Can anyone please give me their best interpretation of $E$ here? Thanks.