If $T_n = \{\frac{a}{n} \mid a \in \mathbb{Z}\}$, then what are $\bigcup_{n\in\mathbb{N}}T_n$ and $\bigcap_{n\in\mathbb{N}}T_n$? 
For each positive integer $n$, let $T_n = \{\frac{a}{n} \mid a \in \mathbb{Z}\}$.
What are $\bigcup_{n \in \mathbb{N}} T_n$ and $\bigcap_{n \in \mathbb{N}} T_n$?

I'm pretty sure the first one is just $\mathbb{Q}$, the rationals. Since the set will have all possible numerators over all possible denominators. The second I'm not so sure of. It's certainly not empty, since $1/2 = 2/4$ so $T_2$ and $T_4$ have non-empty intersection. I am currently leaning towards this set also being $\mathbb{Q}$, since we will get every fraction here as well: if $a/b$ is in $T_b$, then it is also in $T_{2b}$ as $2a/2b$.

You are correct that the union is just $\mathbb{Q}$. As you noted, every pair $(a,n)$ appears in the union and so we have all the rationals. On the other hand, as noted in a comment, the intersection cannot be all the rationals. Clearly the integers are a subset the intersection since every $(mn)/n$ will appear in each $T_n$. Now suppose some rational $p/q$ with $gcd(p,q)=1$ and $q \neq 1$ is in the intersection. Then $p/q$ is not in $T_p$, since $p$ and $q$ are relatively prime (there is no integer $x$ so that $p^2 = xq$ is $p$ and $q$ are relatively prime). Thus, the intersection is simply the integers.
