As I’ve progressed in mathematics, I’ve noticed more and more blackboard bold symbols showing up: $\mathbb{R}$ for the reals, $\mathbb{Q}$ for rationals, $\mathbb{C}$ for complex numbers, $\mathbb{Z}$ for integers, and so on. Some others like $\mathbb{A}$ (affine space), $\mathbb{P}$ (projective space), and even $\mathbb{S}$ (sphere spectrum or sedenions, depending on context) also have established uses.
This got me wondering: do all capital letters eventually show up in blackboard bold in some standard context?
Wikipedia has a list of blackboard bold symbols, and nearly every capital letter is listed there with some mathematical usage—even $\mathbb{X}$ is occasionally used to denote the website formerly known as Twitter a generic metric space. However, I couldn’t find common mathematical meanings for $\mathbb{U}$ or $\mathbb{Y}$ (I also don't think the Wikipedia list has citations for every letter, so I'm not sure how established the mathematical uses given really are).
So my question is:
- Are there recognized, published uses of $\mathbb{U}$ or $\mathbb{Y}$ in mathematics (ideally in textbooks or research papers)?
- More broadly, is there any capital letter that has never been used in a blackboard bold context in any established mathematical literature?
I’m not looking for obscure or one-off notations; ideally, I’d like to know about usages that appear in standard texts or well-cited papers.