Prove that any countable subset of R has empty interior. Is the converse true?
this is one of the question in my midterm that i couldnt do, the prof are not giving out solution, need some help to prove this
Clearly the converse is automatically out. Anything with dense complement is going to have empty interior in $\displaystyle \mathbb{R}$. Any uncountable sets immediately spring to mind? Now, think about it, if $\displaystyle X\subseteq\mathbb{R}$ doesn't have empty interior you can find some $\displaystyle (a,b)\subseteq X$. What's the cardinality of $\displaystyle (a,b)$?