@MartinSleziak To me,
abstract-algebra, even if it were not deprecated, doesn't seem appropriate and could just as well be removed. I suppose
general-mathematics would be a better tag, but it's not clear to me it would be better than simple removal. (Then again, I'm not much of a fan of the question in the first place.) I have no objection though to
general-mathematics as a replacement in this case.
I'm writing an article on Lychrel numbers and some people pointed out that this is completely useless.
My idea is to amend my article with some theories that seemed useless when they are created but found use after some time.
I came with some ideas like the Turing machine but I think I'm not gr...
